I still favor the Sonic Unleashed Final Boss. If I could make a final battle, I would make it so that EVERY character that's ever had a super form battle against this giant monster that can only be harmed in certain ways. For instance, First you play as Super Sonic, do a little damage, but then the monster becomes immune to Sonic's attacks. Then you have to switch to a character that can hurt him. The only characters I can think of are:
Sonic (Super/Hyper/Dark/DarkSpine/Excalibur Sonic)
Tails (Super Tails)
Knuckles (Super/Hyper Knuckles)
Shadow (Super Shadow)
Silver (Super Silver)
Blaze (Burning Blaze)