Chris Rowley

Automatically adapt your embedded videos to the full width of their containers while maintaining the correct aspect ratio.
This is a simple filter that scans each embed in your posts, pages and custom post types, reads the width and height on the <code>iframe</code> and adds a style attribute including CSS <code>width</code>, <code>height</code> and <code>aspect-ratio</code> properties.
Currently supporting YouTube, Vimeo, WordPress/VideoPress and DailyMotion.

Easy Responsive Video

Easy Responsive Video adapts oEmbed video to full width while maintaining aspect ratio.

Palasthotel

greatestview

ATTENTION for WP Version 5+: This plugin does not work with Gutenberg (no problems with Classic Editor plugin though)! Gutenberg brings its own CSS for responsive oEmbed elements, so this plugin would double that and create strange effects.
This plugin uses some minimal CSS rules and a wrapping HTML element to maintain the aspect ratio of oEmbed elements with fixed aspect ratio (e. g. YouTube, Vimeo or Soundcloud).
Unlike other plugins, this plugin does not use any JavaScript!
The aspect ratio is calculated from the (iframe, object or embed) HTML tag width and height attributes. An aspect ratio will only be applied, if both width AND height attributes are given by the oEmbed element and if there is no data-secret attribut set (because those are handled via wp-embed.js). Some oEmbeds have no width or height attributes set, because they calculate their dimension via JavaScript. In those cases this plugin has no effect.
You can find a <a href="https://codex.wordpress.org/Embeds#Okay.2C_So_What_Sites_Can_I_Embed_From.3F" rel="nofollow ugc">list of all oEmbed sites supported by WordPress here</a>.

Responsive oEmbed

Exactly what i'm looking for.

Keep up the good work.

Great Plugin

Makes oEmbed elements with fixed aspect ratio (like YouTube, Vimeo or SoundCloud) scale responsively.

Easy Responsive Video vs Responsive oEmbed