Best JavaScript / HTML5 game engine?

ImpactJS is really good. Currently using that for side-scrolling games. great for indie development.

Worked with LimeJS, but it is based on Closure so a bit hard to get used to. However, it gives great performance on web and mobile.

Playcraft engine (http://getplaycraft.com)

Email us info@playcraftlabs.com and we'll get you on the beta.