Great question! There are some really good examples these days–I catalog them on my blog, Accessibility Wins: https://a11ywins.tumblr.com/
It's a bit unfortunate that it took a legal guideline to get there (the Air Carrier Access Act update), but US airlines are doing a better job with accessibility these days. Alaska Airlines is a great example: https://www.alaskaair.com/
Simply Accessible's website is a fantastic example of accessible RWD: http://simplyaccessible.com/