Assume we're in a Matrix field called postBody which contains different kinds of blocks. videoBlock has three fields: (1) a dropdown for videoService (youtube vs. vimeo), (2) a dropdown for videoAspectRatio (ratio-16-9, ratio-4-3, etc.), and (3) a text field for videoID.

Craft CMS: Video Block

{% if entry.postBody | length %}
<div class="post-body">

	{% for block in entry.postBody %}
	{% switch block.type %}
	
	{% case 'textBlock' %}
	{{ block.text }}
	
	{# cases for other blocks #}
	
	{% case 'videoBlock' %}
	<div class="media-embed {{ block.videoAspectRatio }}">
		{% switch block.videoService %}
		{% case "youtube" %}
		{# see youtube parameters at https://developers.google.com/youtube/player_parameters #}
		<iframe class='youtube-player' type='text/html' src='http://www.youtube.com/embed/{{ block.videoID }}?version=3&amp;theme=light&amp;modestbranding=1&amp;rel=0&amp;fs=1&amp;showsearch=0&amp;showinfo=0&amp;iv_load_policy=3&amp;wmode=transparent' frameborder='0' allowfullscreen='true'></iframe>
		{% case "vimeo" %}
		{# see vimeo parameters at https://developer.vimeo.com/player/embedding #}
		<iframe class="vimeo-player" src="https://player.vimeo.com/video/{{ block.videoID }}?title=0&amp;portrait=0&amp;badge=&amp;byline=0&amp;color=dddddd" frameborder="0" webkitallowfullscreen mozallowfullscreen allowfullscreen></iframe>
		{% endswitch %}
	</div> <!-- media-embed -->

	{% endswitch %}
	{% endfor %} 
</div> <!-- post-body -->
{% endif %}