Sonic Generations has to be one of my top 5 Sonic games ever, no doubt. And I only rank other games above it due to personal taste, but objectively speaking I do see Generations as the most complete and polished Sonic game out there with very little flaws that are irreleventized due to the game's sheer quality.
The game has a very clear focus in it's main loop, which is constant experimentation and improvement in levels for the fastest time, said levels being so well designed for that in both acts 1 and 2 letting you take some crazy shortcuts if you're clever enough. Getting the S-ranks is a bit too generous but at the very least this means getting 100% completion isn't a total pain if you aren't as experienced.
Speaking of 100%, I think this game has the best collectibles as well with the Red Star Rings. They're less hidden around the levels so much as they're laid out in such a way you'll have to explore the whole level to get them, so you get to see different routes you'd maybe never find if it weren't for the Red Rings. (Plus, getting music and concept art from the older Sonic games rocks!)
For the gameplay styles itself, I'll just go ahead and say, I'm very impartial when it comes to Sonic games, if a game is the way it is then that's that and thus I will enjoy it for what it is. That is to say, I don't have a preference for the two gameplay styles in Generations, but even then I think both Classic and Modern Sonic control so well and have such fun levels that they're both really fun despite you preferring one over the other.
Side note on Modern Sonic's jump height, since that was asked about. I can see why it can feel very weak compared to the super floaty jumps of the Adventures games, but really, it never bothered me.
The bosses on a purely gameplay standpoint aren't too much to write home about, but I can't deny they're a spectacle and good fanservice. They're easy to do, even the hard versions, but they're fun to come back to once in a while.
Except the Time Eater.
And to top it off, the extra Challenge Acts that you get to do as a distraction from the main acts, they're not too bad as well. The game forces you to do at least one of them per act, but you can just do the Time-Attack challenge which requires you to just play the super fun levels normally again. But the rest of them are great fun as well, lots of interesting gimmicks that don't take too much of your time like the elemental shields from Sonic 3. It's a bummer you don't get to just play as the characters you've saved because "Sonic's Stupid Friends" and all that, but even then I'd say they're implemented very well for what they are.
Except the Vector one.
The best way I could describe how I feel about Sonic Generations is that, it's not good just for a Sonic game, but it's a good friggin' game in general. If I were to recommend just one game to someone who has no experience with the Sonic franchise, it would be Generations. And it's amazing even for Sonic fans due to all the throwbacks and little extras you can get related to the older Sonic games, it
is and anniversary game after all.
Overall, it's what I've said at the start. I very much prefer other Sonic games, but Generations is still one I'm happy to go back to every time for it's consistent quality all the way through. Genuinely believe to be objectively the best Sonic game to ever come out.
Well, maybe until Sonic X Shadow Generations finally drops, but let's not forget to stay cautiously optimistic with this one.