Without a doubt, this truly is the best Sonic the Hedgehog fangame ever made.
What sets Retro Sonic far above other fangames is principally the Level Development Kit. When you look at the Emerald Hill Zone that Taxman has ported, you can see for yourself just how powerful it really is.
The physics are also the best I have ever seen in any Sonic fangame. They are not quite perfect yet - but without a doubt the physics are far, far better than any Sonic fangame I've ever played that aims to recreate gameplay from the original Sonic games.
The fact that Knuckles and Tails are both playable (with all their moves) is also something that makes Retro Sonic stand above others.
Given that Retro Sonic is currently - by far - the most polished Sonic engine, it is my sincere hope that this game engine becomes the impulse that inspires Sonic gamers and fangame creators alike to break free from the inhibitions of lesser engines and limitations of rom hacking. Like many others, I anticipate that the release of the Level Development Kit and Online Multiplayer will be catalyst to this.
I'm going to produce some constructive suggestions with some comparison video footage and labelled screenshots quite soon to aid the Retro Sonic project. Thank you for your efforts over the last 5+ years Taxman.