My favorite Sonic Game in 2D is Sonic Battle. My favorite 3D Game is Sonic Adventure 2 Battle.

I'm not a Sonic fan by any means, but I did enjoy Sonic 3D Blast quite a bit. I know it's one of the most hated Sonic titles out there, but I thought the change in concept and controls was great, especially the forced defeating of every enemy. Love it.

I'm not the biggest fan of Sonic. I haven't played any of the 2D games but mostly played the 3D games. My favourite 3D Sonic game is Sonic Generations. 

When it comes to classic Sonic games Sonic CD is my favorite I liked traveling to the future and the music was interesting.

My favorite among the modern games is Sonic Adventure, Red Mountain and Lost World are my favorite Zones.

Hmm... For me right now, my favourite Sonic game is probably a toss up between Sonic Battle, Sonic Heroes, and Sonic Generations.
Sonic Battle actually wasn't fully mindless and each character felt different. One of the few Fighting games that I like.
Sonic Heroes... uh... call it nostalgia I guess.
Sonic Generations is mostly for Rooftop Run. That is just such a great stage.

My personal favorites are the first two Sonic games, and Shadow the Hedgehog. I realize it's not traditionally thought of as a "good" game because of some awkward gameplay and how edgy it tries to be, but as a 10 year old it was the coolest thing in the world to me. And because of the memories it created for me as a kid, it's always going to be one of my favorite Sonic games.

definitely sonic the hedgehog 1 because some of the music was awesome

2D is Sonic Battle. 3D is Sonic Adventure 2. I wish they'd make a Sonic game for the later consoles (Xbox 360/PS3/Xbox One/PS4) like Sonic Adventure 2. To be honest Sonic Adventure 2 is probably my favorite game ever. Because of the Chao and the adventuring.

I forgot to mention Sonic Heroes. What a great game. I know 3D Sonic is supposedly bad, but I don't think that's entirely true.

taterii : Yeah, I mean Sonic Adventure and Sonic Adventure 2 are ASTOUNDING games. not to mention Sonic Unleashed and Sonic Generations. Shall I go on?

My most favorite Sonic games is the first 2 Sonic, the reason why is those games are the ones I grew up with, and the other games after doesn't feel the same. The gameplay, the graphics, the sound effects and the music in those games is the best in the series, that's why the forst games are better than recent games. That is why I picked those games as my favorite.

1) Sonic 2
2) Sonic Spinball
3) Sonic and Knuckles

For the 2D sonic games, it's definitely the Sonic 3 and Knuckles combo. Those are well polished, fast, looks good, and is the best of the first 3 sonic games.

3D sonic games have been less appealing to me. I haven't played many of them, but I would say Sonic Adventure 2 is my favorite. However, I am interested in what Sonic Generations and Sonic Colors is like.

   My favorite 2d Sonic game is Sonic 3 and Knuckles. The level design was terrific, and was far and beyond the best of the original 3, especially 1. The game also was the fastest of the original 3, without seemingly punishing you for it. The soundtrack was fantastic as well.
   But a relatively close second place is Sonic Adventure 2. The feeling of speed was better than Sonic 3 and Knuckles, although the game was much less of a challenge, was much shorter, and had less variety. The bosses were also very disappointing in Sonic Advance 2, and the game felt a bit short, and the cop out of making you fight every boss again was not a great way to end it.

   As for 3d Sonic, Sonic Adventure is the only one I played, but I didn't enjoy it much. The controls and camera were not great, I didn't feel like I had much speed in much of the Sonic portion, and actually found that Tails' stages were much more enjoyable. So, i guess it's my favorite 3d Sonic game, but definitely not because I thought it was good.

As far as 2d Sonic goes, I'd have to say my favorite game would have to be Sonic 3 & Knuckles because I feel like the music, levels, and bosses were all better than Sonic 2, which is another fantastic game.

As far as 3d Sonic goes, I'd have to say Sonic Adventure 2 is my favorite because that game introduced Shadow and Big the Cat wasn't in the game YAY!

My favourite 2D Sonic game is Sonic 3 and Knuckles. I played it extensively and it was amazing in every way.

The only 3D Sonic game I played was Sonic Adventure, but it was as good as Sonic 3 and Knuckles.

My favorite Sonic game is actually Sonic Dash which is a mobile runner game. I probably played this game more than I have any other sonic game and I really like it.

Since I have not played beyond the 16-bits platforms, my choice is rather limited. I have to go with Sonic 3 and Knuckles then. The graphics were absolutely amazing for the time and the 3D special stages didn't lag has they did in Sonic the Hedgehog 2. Plus, airlifts from Tails were quite an improvement.

Sonic advance 3 was one of favorite sonic games as a kid. One of the few besides SA2 that were created before the entire series went downhill.

My favourite 2D sonic game has to go to Sonic 2, Mainly because it's the one game that I always seem to go back to the most, and I also have a good nostalgic bond associated to it as well. I mean most of the music is amazing.. Mystic Cave is actually my ring tone because it's awesome.. Also it was the introduction of probably my favourite Sonic character which is Tails..

My favourite 3D sonic game is probably Sonic Generations, Mainly due to the fact that it felt like playing some of the classic games whilst actually being modern and pretty entertaining to play.. I think nostalgia may have played in my part of the Enjoyment of it to be fair.... Though I did like most of the bosses and the different amount of routes you could take... 

Plus trying to see who could get the furthest in a level was always fun x3