Sonic 2 Delta - A rather fun Sonic the Hedgehog 2 ROM hack that I personally consider better than the original. It's primary goal is to restore some cut out Zones from the game (such as Hidden Palace, Wood Zone, and Dust Hill), but also adds some cool new features like Sonic 3's file select screen. It even includes an entirely new character combination - Knuckles & Tails!
The most recent update has added all of Sonic 1's Zones (due to the Nick Arcade prototype's inclusion of them), so basically it's Sonic 1+2. My only real complaint is that some of the restored Zones feel a bit meh design wise (Winter Hill Zone has a really odd layout), but it's otherwise great fun and the other Zones are worth taking a look at. I consider it much better than the original, with only the Android remake by Taxman surpassing it.
Newer Super Mario Bros. Wii - Basically "New Super Mario Bros. Wii As You Imagined It". It contains a much less bland sounding soundtrack, much more varied level art, and makes New Super Mario Bros. Wii a lot more fun. The core of the game is great, but the original game's level design and music just left me feeling kind of unsatisfied. For whatever reason, it felt like it wasn't trying as hard as the original DS title did. This fixes that.
Melee 20XX - A hack that has more recently popped up. Includes a lot of nice options such as being able to toggle the debug menu, and the trophy stages being accessible in normal Smash modes. I'd highly recommend giving this thing a look if you're curious about running Melee competitively, since this includes a lot of nice options to help out newcomers.
Project M - Basically Brawl but much more balanced and more additional goodies such as more skins. Incredible for what it is.
Since Doom was mentioned, if you're looking for a really fun more-vanilla like experience, a WAD popped up a while ago called Smooth Doom. It's a WAD that's only compatible with ZDoom-based source ports, and it updates every monster and the player's sprites to have much more fluid animation. Combine that with GZDoom and it looks pretty nice. Very fun if you want to re-experience Doom while still maintaining some familiarity.
The Eternity Engine is also kind of cool on the topic of Doom. It's a source port that focuses on porting over some features Quake had and tries to make Doom a bit more "modern". The end result is a Doom source port that can have a lot of nice eye candy for you to take a look at.
