Let's begin discussing; May I just remind you to keep it a bit rational as when discussing "What to improve on sonic" It's probably one of the worst topics in the entire industry.
Personally my favorites with the Genesis/Game Gear/CD era were Sonic CD, Sonic 3 & Knuckles, and my favorite of those three containing my favorite VG character Nack the Weasel.
As for 3D games I only really liked Sonic Adventure 1/2. The others I've played were from decent at breast I mean best. Though I have yet to try Sonic Unleashed which is apparently the best 3D game after those too.
And of course we have sonic's future title...Sonic 4. It's actually what? Another 2 months? Not to shabby.
And I'll leave improvement for you all to rip apart for starting with.
Sonic Unleashed was enjoyable for the most part. The day levels are the good half of the game. The final boss is like a (good) Punch-Out knock off until you go super saiyan.
Sonic 4 looks good from what I can tell from the screen shots.
Genesis games were great.
Sonic adventure 1 and 2 were great
Sonic heroes was decent
Sonic unleashed was pretty good.
The rest is quite poopty.
Have high hopes for Sonic 4.
The only time I've ever played the classic games is when I sucked ass on them, which made me not like them very much, so I only have real opinions on the 3D games I've played.
Sonic Adventure 1 DX was AWESOME
Sonic Adventure 2: Battle was AWSOMER
Sonic Heroes would have been good if every level for every team wasn't the same level in the SAME ORDER and to get the Last part you didn't have to get the Chaos Emeralds in the way you had to...
Sonic 06 I actually liked, I mainly didn't like the loading screens and the fact that Shadow had weapons/cars.
Sonic &tSR sucked so much ass, because there were some levels you had to do that were RETARDED. "Break all the jars." <-what the intercourse is that? I also hated how he just kept running and running and you couldn't explore.
Sonic Rush was pretty fun, again hated how you had to get the Chaos Emeralds.
Sonic RPG (I forgot it's real name, it was for the DS) was ultra meh, could've been a lot better, I kinda hope to see some of the characters introduced in it to return.
As for the non-adventure games
Sonic Riders was meh, good idea on paper, not as good when played.
Sonic Riders: Zero Gravity was the same as the above. I hope to see the characters introduced in these games to appear in a real game sometime, not as a main character, but as a part in the story, and his levels are mainly riding his gear, not in a race, just to get somewhere, then continued on by everyone else.
If I didn't mention a game I either haven't played it or didn't think of it.