What's the most memorable Sonic game to you, and why? I'd have to go with Sonic Heroes. It's the first Sonic game I've ever played. I came across a PC demo for the game and had to try it out. I really enjoyed it at the time. Then a few months later I picked up the full version, which then got me into checking out the other titles in the series. I have a lot of good memories of playing the game too. Something I can't say I've had a lot of with the other Sonic games in the series.
It's not my favourite Sonic game, but I'd have to say it is the most memorable experience I've had with a Sonic game so far.