I bought some secondhand games for my Sony PS2 a few weeks back, including a number of Sonic the Hedgehog discs, and I haven't been able to stop playing the classic Sonic Heroes game from 2004 since - it's still utterly brilliant!!
If you haven't got it even after all this time, you must go and get yourself a copy. It's the way a Sonic game should be - fast and furious fun. Why oh why can't Sega go back to that classic style instead of all these sad "pseudo" Sonic games they've been churning out over the past few years? Oh dear!
You can play a proper classic Sonic the Hedgehog game online for free here: Ultimate Flash Sonic Online Game