Caveat: This worked for me, but might not for you. If you take any of my advice and something goes horribly wrong, you are entitled to a full refund of goose eggs.
TL;DR: For a 2-floor home, with a mix of 5+ year-old wifi devices, to reasonably new iOS, Macs, & set top boxes, and 22+ competing network signals, the best mix I found was:
Central 2nd floor wifi router, 5Ghz band set to a fixed (not auto) channel, 802.11a/n, with 40Mhz width produced optimal speed and reliability, delivering 78-145 Mbps down and ~25 Mbps up on a ~200Mbps connection. 2.4Ghz band set to 802.11b/g/n with 20Mhz width for old devices or extended range. The specific channels used should be entirely based on signal maps using the apps mentioned below. Sit where you and your friends/family sit (in bed, on the sofa, at the table, on the floor, wherever, and take several samples to find the least congested channels).
The 5Ghz setup was the best setting for every nearly device (Nest, Roku 3, iPad 3rd gen, iPod 5th gen, and iPhone 6