For the record, it's not just Sonic 1-3; Sonic & Knuckles was great as well and stands up with the original 3. The original isn't as great in light of how good its successors were. Also Sonic Spinball was a really fun game with a cool aesthetic. And Sonic 3D was an interesting take on 3D. It felt more like a demo of things to come, but I like how it was built up of all these different areas to solve, it gets a good grade in my book.

But the fact that all of the above are Genesis games may give you the answer - it's all just rose tinted glasses.

To me, what ruined the series is how the developers looked at the speed aspect that set it apart from the Marios and Mega Man's, and then made it the ONLY thing. Even when Sonic Unleashed was being marketed, even when they were only showing the "classic" parts, I was exceptionally turned off by the game. You just fly through the levels with no real concentration at all. Yes, there are parts where you race through the loop-de-loops, but in the original Sonic games there are ALOT of vertical parts and ALOT of parts where you're not moving fast at all, maybe just the opposite b/c you're trying to balance your way across the platforming segments.

I do believe the originals were masterpieces and not just "very well done" games, which is something I would reserve for, say, Shining Force (agreeing with Vyk). The sense of inertia (something Sonic 4 lacks) differentiated it a bit. The music is near untouchable as far as I'm concerned. It embodied the Sega Genesis experience - a lot of original-looking, highly detailed crap that moves really really fast. With unbelievable music.

Sonic 4 is good, but it's not that good.