(OKAY, I KNOW HOW LONG THIS IS, BUT AFTER SPENDING LIKE A HALF HOUR TYPING ALL THIS I'M NOT JUST GONNA DELETE IT. DEAL WITH IT.)
3. Sonic 2.
While Sonic 3 was technically a better game, Sonic 2 was the better sequel. Allow me to explain.
Sonic 2 added so many staples to the series it's not even funny. I mean really, it's so hard to play Sonic 1 soon after playing 2 because the spin-dash is just so useful and so... well, Sonic. And Tails, one of the most popular characters in the entire series debuted in this game. On top of that, while Sonic 1's precision platforming had it's charm, 2 realized the point of the series' gameplay: Speed. Pure, unadulterated speed. While this made the game easier in comparison, it was so much more fun. Not meaning to throw Sonic 1 under the bus here, I'm just saying Sonic 2 was just a huge improvement and one of the best examples of a sequel I've ever seen.
2. Sonic Generations.
This game came out in a time the series needed it. Albeit a little late, but still. Before this game was out, Sonic was basically the laughing stock of the entire industry with the absolute trainwreck named Sonic '06 being followed by titles that were good but didn't bring the revival that the series needed. (Unleashed's day-time stages were some of the best of the series, but the game as a whole was bogged down by the Werewolf levels, and Colors was decent, just not good enough to save the series at the time.) When this game came out though... not only was it great on it's own merits, but it was exactly what the series needed to get on it's feet: a well made, well-polished compromise between classic Sonic fans and modern Sonic fans. With the level designs and soundtrack rivaling Unleashed with the tightened controls and lack of werehog stages (there was even a fan-made Unleashed mod released later on PC that took the day-time levels and put them in Generations complete with the fine-tuned controls, which I would nominate for this spot but it's not really a standalone official Sonic game so, hey, what can you do?), the series was finally back on it's feet and in full-force until the slight disappointment of Lost World but I mean it had some pretty big shoes to fill so I don't blame it.
1. Sonic Adventure/Sonic Adventure 2
I'm sorry... I'm just sorry, I really can't decide which game to put here, they are both equally amazing. Let's just start with Adventure.
Adventure's case is similar to Generations, the series wasn't seeing a lot of action during the whole 3D craze with 3D Blast falling flat on it's face and Sonic R not exactly winning any medals either (
Warning: Language). Clearly, Sonic was having some trouble entering the third dimension here. So when Adventure came out, it was really a sight to behold. I'm not gonna go into too much detail about everything right with it, if I did you'd be here for a while, but let's just say: The controls were absolutely amazing, some of the best controls in a Sonic game to this very day, and E-102 Gamma is my most favorite Sonic character PERIOD. The bosses in his story sucked (save for the final boss, that one was awesome), but that ending... so many feels. Just too many to count.
Moving onto Adventure 2, it did some things better than Adventure, though at the same time some things worse. Not gonna touch on the bad, so let's just praise what this game did right: Shadow was introduced and before Heroes he was actually a really good character, the speed stages are a lot of fun and I can play through any of them at any time, the soundtrack is ABSOLUTELY AWE-INSPIRING (except for Knuckles. The great emerald's power allows me to feel?), the character animations were extremely better than... uh... whatever's going on over here:
| and the Chao Garden was improved as well.