I personally must +1 the electric / lightning shield because of both the ring magnetism and the double jump it added allowing access to secret or harder to reach places / shortcuts. I think if I had to choose one it would be the lightning shield, but with that said, all of the shields in Sonic 3 & Knuckles were pretty kick ass and added a nice game changing element. I also just realised that the reason I like the boost in modern games is because of the ring magnetism.
This is true. I only use Knuckles in Sonic Jump for just that reason. Seems odd though, like why didn't they give Tails the higher jump power and Knuckles the lower one?
Can I be the first to say the roll? I mean, especially in games like Sonic 3 and Knuckles, or Sonic Advance 1 and 2 (haven't really played 3, so I can't judge), whenever I'd use the roll well, it'd give me this sense of speed that kind of just feels natural and integrates well into the level in most cases. I've been playing Advance 2 recently, and it's honestly one of my favorite moves there. Apart from the roll, I really appreciate moves like the bounce attack from Sonic Adventure 2. While I think today I'd prefer the stomp, I've liked the SA2 bounce attack if only for the slight enjoyment I get from bouncing, and that it maintains some horizontal velocity while bouncing, unlike a certain Sonic 06.
The homing attack circa Sonic Adventure 1 was pretty good in that It served double duty as both an actual attack and an effective speed boost. The way it works now kind of stilts the fluidity of modern Sonic games IMO.