The Sonic series after the Sega Genesis/Megadrive:
A tier:
B tier: Sonic Advance 1, Rush
C tier: Sonic Advance 2, Sonic Advance 3, Rush Adventure
D tier: Adventure, Adventure 2, Heroes, Unleashed, Secret Rings, Sonic 4
F tier: Shadow the Hedgehog, Sonic 2006
A tier is a great game.
B is above average and worth playing.
C is average.
D games are not very good and usually suffer from lots of filler and/or poor design choices. There may be some decent aspects buried deep in the game, but its not worth the time or effort to find it.
F is broken in all aspects plus featuring horrible artistic and design choices.