Wellllllll, Sonic Colors actually made me start clicking on Sonic threads, so there's that.
I think Sonic 1 and whatever Game Gear Sonic game I had are decent. Sonic 2, 3, and Knuckles are good, as are most of the GBA/DS handheld titles. I think Sonic Colors is great, and Generations is slightly below Colors for me.
One of the reasons I disliked Sonic growing up is because I felt like you'd need to put in a lot of effort to get the most out of it. You'd have to memorize stages since you're moving so fast and experiment to find the best way through things. It was very different than most other platformers I played where obstacles would be right in front of you and plain to see. The satisfaction in Sonic wasn't surviving--it was the rush you got from performing flawlessly. I didn't find that type of play fun until later on. I much preferred traditional, weightier platformers like SMB3 (and the less weightier SMW) and action platformers like Mega Man X. I never found early Sonic as fun to play those and still don't. I'm very, VERY picky about how characters 'feel' in these sorts of games. For example, I loathe how NSMB Wii feels, but I appreciate the majority of its level design.
Adventure and Adventure 2 (and other 3D Sonic games that include some that shall not be named) started putting in all this filler that threw me off. I fucking hate filler. Just give me a stage select and the option to skip cutscenes. I also don't think the transition to 3D space worked out too well, either, as the series started relying too much on context sensitive prompts. Not having proper camera control also bothered me. And if I hear Chao Garden one more time... I was there with Sonic Jam for Sonic's first foray into 3D (unless I'm mistaken, or unless you count 3D Blast, which I also had the misfortune of owning), and I've watched those infamous LPs all the way through, enough to see the genesis of modern 'Burnout' Sonic in Unleashed.
But I watched those after I played Colors. I had all but ignored Sonic until Colors had been out for a while. I got it on a whim. It was a revelation. It's my favorite Sonic game so far. I won't go into details about why I think the meaningful variety of wisps, boosting and meter management, large, explorable levels, an actually decent scoring system, amazing music, okay bosses, and mostly innocuous story all come together so well, but I will say that I adore what Sonic has become. I like it so much more than what I've played before. It's not as stupidly simple, boring, luck-based, or shitty. There is now enough depth to Sonic (and pleasure derived from controlling him) to make speedrunning feel as rewarding as it should've been years ago to me. Colors also has my absolute favorite underwater stages in any game that changes up its mechanics underwater (yes, any game--out of everything I've played in my entire life). Sonic can endlessly jump ('swim'), immediately return to the ground with a stomp from any height, get decent speed from simply running on the floor, boost anywhere (even boost across the surface), and use a yellow wisp to quickly fly right through the stage. And the final boss is actually a creative twist on the game's mechanics. It's a very polished game. I'm sure there are physics snobs who don't like how Sonic moves, but, for what the game requires you to do, everything is adequate and more functional than it has ever been. You even have an honest-to-goodness double jump.
Generations...is still really good but pushes more obnoxious content (some horrible story stuff and awful challenges) and changes a few things about 'modern' Sonic. He can't jump off any wall anymore (sad), swim (sad), milk points (sad), or use all the wisps found in Colors (sad), but the way you drift (I think this is the term) is more conducive to speeding through levels. 'Classic' Sonic is okay but not the highlight of that game for me. In a couple of challenges you have access to the old elemental shields, which should've been integrated better in the main stages, that make him more fun. The ability equip system (I forget what it's called) is also nice, offering players a way to tweak how they want Sonic to control. The music is understandably great, too. I am still sad that Colors' representation was really weak since Planet Wisp is a slog in Generations. I love that the game was released on PC because performance on consoles is not the best.
I think Sonic is on the right path right now. Colors and Generations are two platformers that offer something not many other games have been offering lately. The handheld games have been doing their own thing that people seem to enjoy, too. The closest thing we have to this 'modern Sonic' now is probably a hybrid between BIT.TRIP RUNNER, Mirror's Edge, and Burnout, but even those aren't quite the same (especially when power-ups and exploration are factored in). There are people who still love the older games, and that's totally fine, but I don't think anything new will satisfy them, whether or not Sega actually tries to or succeeds in appeasing their demands. As someone who was never particularly attached to the earlier games, I couldn't care less. I think it's unfortunate that the series has the stigma it brought upon itself, and I'm sad that some people who gave Colors and Generations a try aren't enjoying the games as much as others. I don't want this style of game to die off like so many other endangered genres right now (quality Metroidvania is practically on life support).
tl;dr I never was a fan of Sonic until Sonic games stopped being like Sonic games. My only issues now are shoehorned story, filler, and awkward design choices that get in the way of the franchise's unique brand of speedy 2D platforming.