Sonic Flash was my introduction to the Sonic games and I'd say that was pretty good.
Other than that, I remember loving a turn-based 2D stick-figure shooter that I don't remember the name of but was pretty fun since there were lots of weapons, multiple maps, ways to interact with the environment and multiplayer to play with my brother.
And finally, Armed with Wings which was a really fun 2D hack-n-slash with an unique artstyle by being all silhouettes.
It's also what introduced me to Stickerbrush Simphony lol