Number Five- Sonic UnleashedSonic Unleashed, for the Wii, PS2, PS3, Xbox 360, and probably some other console I'm forgetting, was typically considered to be a mediocre or even bad Sonic title. I, however, really enjoyed it. The day levels playing as normal Sonic were done pretty well in my opinion, and the last day level of the game is known to be one of the hardest levels ever in any Sonic game. I honestly found the night levels to be really fun, and the game had a really good way of surprising you with swarms of enemies, or even mini-bosses that look rather menacing. I also enjoyed the game's music, in fact it's one of my favorite soundtracks in the series. I'm putting the game as low as Number 5 because the controls seemed a bit awkward at times, and I really wasn't too interested in the game's (rather bland) story.
Number Four- Sonic ColorsSonic Colors, released exclusively on the Wii and DS, and the next main series game after Unleashed, was seen as a pretty good game in the Sonic Community. And honestly, I can see why. This game fixed almost EVERY flaw that I had with Unleashed. The Controls were great, even giving you the ability to use a GameCube controller, and the story was creative with cheesy humor I found quite entertaining. The level designs were much more creative, and there was a lot of focus on areas with a 2D camera angle. Colors also added some Mario-esque powerups in the form of Wisps, small Alien creatures that Sonic can absorb because WHY NOT. Each area had 6 levels and a boss, except for the final one. You also hear Eggman saying REALLY bizarre and quite hilarious messages at random times throughout the levels, which really help with the game's less serious story. The music in the game was pretty good, but definitely not my favorite. Other then that, the only reason Colors is in Number 4 is because I just like other games more than it.
Number Three- Sonic Adventure 2Sonic Adventure 2, for the Dreamcast and later ported to GameCube, Xbox 360, and PC through Steam, was the sequel to Sonic Adventure, which (omg spoylurz) is next on this list. The game offered 6 playable characters, however they were actually just hero/evil counterparts to each other so there was really only 3 different styles of play. The ability to play as both the Heroes and the villains was a really good concept, but the identical play styles made it seem a bit less awesome. The level design in SA2 was arguably some of the best in the series, and the music was great too, especially Knuckles' insanely cheesy rap songs. It also had the Chao Garden, a virtual pet area, which I will confess I spent probably more time in then in the game's actual levels. Sonic Adventure 2's story is also my favorite in the series by FAR, because it was more than just "omg egmanz takkin ovr da worl stop he" which was quite repetitive at that point. Back to the gameplay, though. Playing as Sonic/Shadow was pretty fun, going through the fast-paced worlds, and also having an area to explore for the secrets that you wouldn't notice if you were just speeding through the level, such as an area in Sonic's last level that let you skip about 5 minutes of the level. Playing as Tails/Eggman was probably my least favorite, you were in a rather slow mech and had to go through levels, shooting enemies with your lasers. The platforming in these levels was basic jumps that don't really require skill, and the main reason to play these levels more then once is to get items for the previously mentioned Chao Garden, as these levels are the best place to get them. Playing as Knuckles/Rouge was pretty fun, you had to go around these somewhat small areas looking for items, but where they were was random, giving them a ton of replayability.
Number Two- Sonic AdventureSonic Adventure! Released for all the same platforms as SA2, this game was glorious. It introduced the Chao Garden, which was ok but nowhere near as good as Sa2's. It had amazing music, a basic yet still unique story, and 6 different characters to play as! First you have Sonic, who has really fun platforming levels. Then you have Tails, who ALSO has fun platforming that's more nonlinear than Sonic's. Next there's Knuckles, who plays much like he does in Sa2. There's also E-102 Gamma, who plays like Tails/Eggman in Sa2, but with more platforming and time limits that make each level much more tense. Then there's Amy, who spends the whole game running through somewhat fun puzzle levels while being chased by a robot for some reason. Lastly there's Big the Cat, who spend the whole game... Fishing. FISHING. FISHING IN MY GAME ABOUT FAST ANTHROPOMORPHIC BLUE HEDGEHOGS. AHHHHHH. Actually, unlike most of the Sonic Community, I adored the peaceful fishing levels because why not. Honestly, the main reason I prefer Sonic Adventure to its sequel, is because of a lot more variety in styles of gameplay.
And Finally, Number One...

Sonic 3 And KnucklesYes! Sonic 3 And Knuckles for the Sega Genesis/Mega Drive was the "full" version of Sonic the Hedgehog 3, and was split into 2 different, combine-able (that's not a word is it ;;) games due to Time constraints. Honestly, I couldn't have chosen another game. Not only is this my favorite Sonic game, but it's one of my favorite games in general. This game DEFINED my idea of a perfect platformer, fast-paced but with enough enemies and traps to stop you from just running through each level insanely fast. Each level has its own gimmick or set or gimmicks, from water to ghosts to zero-gravity. this game is by no means the best Sonic Game technically, but both nostalgia and my absolute love for side-scrolling platformers skyrocket it into Number One.
So there you have it! my top Five Favorite Sonic Games.