[Updated! Aug 14 2020] YouTube recommended encoding settings on ffmpeg (+ libx264)

README.md

Parameters

Container: MP4

Parameter	YouTube recommends setting
-movflags faststart	moov atom at the front of the file (Fast Start)

Video codec: H.264

Parameter	YouTube recommends setting	Note
-c:v libx264	H.264
-profile:v high	High Profile
-bf 2	2 consecutive B frames
	Closed GOP	x264 produces closed gop as default
-g 30	GOP of half the frame rate.
	CABAC	Normally, x264 uses CABAC as an entropy encoder.
-crf 18	Variable bitrate.	Currently, YouTube does not have a bitrate limit. Refer official document: https://support.google.com/youtube/answer/1722171?hl=en
-pix_fmt yuv420p	Chroma subsampling: 4:2:0	If you like to encode HDR video, this comment might be helpful. https://gist.github.com/mikoim/27e4e0dc64e384adbcb91ff10a2d3678#gistcomment-2859601

Audio codec: AAC-LC

Parameter	YouTube recommends setting	Note
-c:a libfdk_aac -profile:a aac_low	AAC-LC	Fraunhofer FDK AAC is a high-quality codec library, but it does not compatible GPL. So your FFmpeg most likely does not contain it. Build FFmpeg manually or use a built-in AAC encoder instead of libfdk_aac.
-b:a 384k	Mono 128 kbps, Stereo 384 kbps, 5.1 512 kbps

Sample video

• https://youtu.be/UCTPyB0l37Y

References

• Recommended upload encoding settings - YouTube Help https://support.google.com/youtube/answer/1722171
• Encode/H.264 – FFmpeg https://trac.ffmpeg.org/wiki/Encode/H.264
• Encode/AAC – FFmpeg https://trac.ffmpeg.org/wiki/Encode/AAC
• x264 FFmpeg Options Guide - Linux Encoding https://sites.google.com/site/linuxencoding/x264-ffmpeg-mapping

encode.log

ffmpeg version N-98725-gcfc6552032 Copyright (c) 2000-2020 the FFmpeg developers
  built with gcc 10 (GCC)
  configuration: --prefix=xxx --enable-nonfree --enable-gpl --enable-libfdk-aac --enable-libx264 --enable-libx265
  libavutil      56. 58.100 / 56. 58.100
  libavcodec     58.100.100 / 58.100.100
  libavformat    58. 50.100 / 58. 50.100
  libavdevice    58. 11.101 / 58. 11.101
  libavfilter     7. 87.100 /  7. 87.100
  libswscale      5.  8.100 /  5.  8.100
  libswresample   3.  8.100 /  3.  8.100
  libpostproc    55.  8.100 / 55.  8.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'source.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf58.20.100
  Duration: 00:00:23.60, start: 0.000000, bitrate: 13088 kb/s
    Stream #0:0(eng): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 3840x2160, 12970 kb/s, 23.98 fps, 23.98 tbr, 24k tbn, 47.95 tbc (default)
    Metadata:
      handler_name    : ?Mainconcept Video Media Handler
    Stream #0:1(eng): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 129 kb/s (default)
    Metadata:
      handler_name    : #Mainconcept MP4 Sound Media Handler
Stream mapping:
  Stream #0:0 -> #0:0 (h264 (native) -> h264 (libx264))
  Stream #0:1 -> #0:1 (aac (native) -> aac (libfdk_aac))
Press [q] to stop, [?] for help
[libx264 @ 0x2337f40] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX FMA3 BMI2 AVX2 AVX512
[libx264 @ 0x2337f40] profile High, level 5.1, 4:2:0, 8-bit
[libx264 @ 0x2337f40] 264 - core 159 r2999 296494a - H.264/MPEG-4 AVC codec - Copyleft 2003-2020 - http://www.videolan.org/x264.html - options: cabac=1 ref=5 deblock=1:0:0 analyse=0x3:0x113 me=hex subme=8 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_range=16 chroma_me=1 trellis=2 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=-2 threads=27 lookahead_threads=4 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=2 b_pyramid=2 b_adapt=1 b_bias=0 direct=3 weightb=1 open_gop=0 weightp=2 keyint=12 keyint_min=1 scenecut=40 intra_refresh=0 rc_lookahead=12 rc=crf mbtree=1 crf=18.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
Output #0, mp4, to 'output.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf58.50.100
    Stream #0:0(eng): Video: h264 (libx264) (avc1 / 0x31637661), yuv420p, 3840x2160, q=-1--1, 23.98 fps, 24k tbn, 23.98 tbc (default)
    Metadata:
      handler_name    : ?Mainconcept Video Media Handler
      encoder         : Lavc58.100.100 libx264
    Side data:
      cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: N/A
    Stream #0:1(eng): Audio: aac (libfdk_aac) (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, s16, 384 kb/s (default)
    Metadata:
      handler_name    : #Mainconcept MP4 Sound Media Handler
      encoder         : Lavc58.100.100 libfdk_aac
[mp4 @ 0x2346280] Starting second pass: moving the moov atom to the beginning of the file0.881x    
frame=  565 fps= 20 q=-1.0 Lsize=   50496kB time=00:00:23.57 bitrate=17548.0kbits/s speed=0.828x    
video:49380kB audio:1107kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.017619%
[libx264 @ 0x2337f40] frame I:54    Avg QP:12.52  size:145222
[libx264 @ 0x2337f40] frame P:508   Avg QP:15.73  size: 83671
[libx264 @ 0x2337f40] frame B:3     Avg QP:15.72  size: 72715
[libx264 @ 0x2337f40] consecutive B-frames: 98.9%  1.1%  0.0%
[libx264 @ 0x2337f40] mb I  I16..4: 32.3% 61.1%  6.6%
[libx264 @ 0x2337f40] mb P  I16..4: 18.7% 32.0%  0.7%  P16..4: 21.4%  3.8%  1.2%  0.0%  0.0%    skip:22.1%
[libx264 @ 0x2337f40] mb B  I16..4:  4.4% 10.4%  0.4%  B16..8: 48.1%  7.4%  0.4%  direct: 3.5%  skip:25.4%  L0:65.6% L1:33.4% BI: 1.0%
[libx264 @ 0x2337f40] 8x8 transform intra:62.0% inter:85.3%
[libx264 @ 0x2337f40] direct mvs  spatial:33.3% temporal:66.7%
[libx264 @ 0x2337f40] coded y,uvDC,uvAC intra: 25.4% 34.5% 3.6% inter: 5.2% 15.3% 0.0%
[libx264 @ 0x2337f40] i16 v,h,dc,p: 11% 21%  8% 60%
[libx264 @ 0x2337f40] i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 30% 18% 23%  4%  5%  5%  6%  5%  5%
[libx264 @ 0x2337f40] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 32% 16% 18%  5%  8%  7%  6%  4%  4%
[libx264 @ 0x2337f40] i8c dc,h,v,p: 56% 22% 17%  5%
[libx264 @ 0x2337f40] Weighted P-Frames: Y:0.0% UV:0.0%
[libx264 @ 0x2337f40] ref P L0: 72.7%  1.1% 15.9%  6.0%  2.6%  1.7%
[libx264 @ 0x2337f40] ref B L0: 89.1%  7.4%  3.5%
[libx264 @ 0x2337f40] kb/s:17165.92

encode.sh

#!/bin/bash

AAC_ENCODER=libfdk_aac
# AAC_ENCODER=aac

AUDIO_PARAMS="-c:a $AAC_ENCODER -profile:a aac_low -b:a 384k"
VIDEO_PARAMS="-pix_fmt yuv420p -c:v libx264 -profile:v high -preset slow -crf 18 -g 30 -bf 2"
CONTAINER_PARAMS="-movflags faststart"

# You need to adjust the GOP length to fit your source video.
# 60 fps -> -g 30
# 23.976 (24000/1001) -> -g 24000/1001/2  (???) <- plz comment

ffmpeg -i "$1" $AUDIO_PARAMS $VIDEO_PARAMS $CONTAINER_PARAMS "$2"

mediainfo-output-mp4.log

General
Complete name                            : output.mp4
Format                                   : MPEG-4
Format profile                           : Base Media
Codec ID                                 : isom (isom/iso2/avc1/mp41)
File size                                : 49.3 MiB
Duration                                 : 23 s 616 ms
Overall bit rate                         : 17.5 Mb/s
Writing application                      : Lavf58.50.100

Video
ID                                       : 1
Format                                   : AVC
Format/Info                              : Advanced Video Codec
Format profile                           : [email protected]
Format settings                          : CABAC / 5 Ref Frames
Format settings, CABAC                   : Yes
Format settings, Reference frames        : 5 frames
Format settings, GOP                     : M=1, N=11
Codec ID                                 : avc1
Codec ID/Info                            : Advanced Video Coding
Duration                                 : 23 s 566 ms
Bit rate                                 : 17.2 Mb/s
Width                                    : 3 840 pixels
Height                                   : 2 160 pixels
Display aspect ratio                     : 16:9
Frame rate mode                          : Constant
Frame rate                               : 23.976 (24000/1001) FPS
Color space                              : YUV
Chroma subsampling                       : 4:2:0
Bit depth                                : 8 bits
Scan type                                : Progressive
Bits/(Pixel*Frame)                       : 0.086
Stream size                              : 48.2 MiB (98%)
Writing library                          : x264 core 159 r2999 296494a
Encoding settings                        : cabac=1 / ref=5 / deblock=1:0:0 / analyse=0x3:0x113 / me=hex / subme=8 / psy=1 / psy_rd=1.00:0.00 / mixed_ref=1 / me_range=16 / chroma_me=1 / trellis=2 / 8x8dct=1 / cqm=0 / deadzone=21,11 / fast_pskip=1 / chroma_qp_offset=-2 / threads=27 / lookahead_threads=4 / sliced_threads=0 / nr=0 / decimate=1 / interlaced=0 / bluray_compat=0 / constrained_intra=0 / bframes=2 / b_pyramid=2 / b_adapt=1 / b_bias=0 / direct=3 / weightb=1 / open_gop=0 / weightp=2 / keyint=12 / keyint_min=1 / scenecut=40 / intra_refresh=0 / rc_lookahead=12 / rc=crf / mbtree=1 / crf=18.0 / qcomp=0.60 / qpmin=0 / qpmax=69 / qpstep=4 / ip_ratio=1.40 / aq=1:1.00
Language                                 : English
Codec configuration box                  : avcC

Audio
ID                                       : 2
Format                                   : AAC LC
Format/Info                              : Advanced Audio Codec Low Complexity
Codec ID                                 : mp4a-40-2
Duration                                 : 23 s 616 ms
Bit rate mode                            : Constant
Bit rate                                 : 384 kb/s
Channel(s)                               : 2 channels
Channel layout                           : L R
Sampling rate                            : 48.0 kHz
Frame rate                               : 46.875 FPS (1024 SPF)
Compression mode                         : Lossy
Stream size                              : 1.08 MiB (2%)
Language                                 : English
Default                                  : Yes
Alternate group                          : 1

-ar 44100 \

Most results from https://html.duckduckgo.com/html?q=Best%20ffmpeg%20flags%20for%20YouTube say to use AAC-low with -ar 44800.
Due to https://wikipedia.org/wiki/Nyquist%E2%80%93Shannon_sampling_theorem

If a function x ( t ) {\displaystyle x(t)} contains no frequencies higher than B hertz, then it can be completely determined from its ordinates at a sequence of points spaced less than 1 / ( 2 B ) {\displaystyle 1/(2B)} seconds apart.

A sufficient sample-rate is therefore anything larger than 2 B {\displaystyle 2B} samples per second. Equivalently, for a given sample rate f s {\displaystyle f_{s}}, perfect reconstruction is guaranteed possible for a bandlimit B < f s / 2 {\displaystyle B<f_{s}/2}.

both 44100 and 48000 hertz are more than twice the 20000 hertz limit of humans, but if YouTube uses 48000 hertz (and you want to prevent recompression) you should use 48000 hertz.