New Tricks, a mod that adds extra abilities to characters like Tails, Amy, and even Knuckles.
Anything from MainMemory is a good choice, especially the Physics Swap mod so you can make characters like Amy feel better by giving her Sonic's physics.
SADX Chao World Extended adds some SA2 features to SA1's garden, if you like Chao.
SADX Heroes, a total conversion mod that lets you play Sonic Heroes levels and characters in SA1.
SA2 Conversion Mod, a total conversion mod that lets you play SA2 in SA1
SADX Chao Gameplay lets you take Chaos into levels with you, and adds them directly to the main gameplay! Haven't messed with it yet myself but it's super impressive.
There are also full restoration of cut beta stages that you can download as well from the SADX Installer's offerings.