A simple questions to all Sonic fans.

[Vipershark]

The entire concept of Knuckles was really cool.
He took all of Sonic's best features and preserved them, while adding in new ones of his own.
The gliding and wall climbing were a natural fit.

I like Super Sonic/Hyper Knuckles/Turbo Tails (and their level 2 master emerald variants)

I really like the spindash, but only when done correctly.
Sonic CD style spindashing and Sonic Generations do not do it right.
Sonic 2 - 3K are how spindashing should be.

I know they're not classic Sonic games, but I found the powerups and item upgrades from the Adventure games to be really cool.
Getting new abilities later in the game and then being able to go back to earlier levels and access new paths was cool.