So many to pick from but here's my favourites:
Rainbow Islands - Simply the greatest arcade style platformer ever!
Superfrog - Best "massive" platformer. Often I get bored with huge levels and can't be bothered finding the exit but I don't find this in Superfrog at all.
Giana Sisters - Surely the game most people say "does it have one or 2 N's in the name?" Perfect on the C64, alright on the Amiga.
Rodland - Perfectly cute platformer by the SWIV programmer.
Ghosts'n'Goblins - Perfect difficulty level on C64. Superb gfx on the Amiga version but has an annoying delay every few seconds where it just locks up for a while.
Creatures - Super on the C64, the Amiga version is naff.
Turrican 2 - Such an awesome game, perfect graphics, music, playability and yet some people dislike it. They must be mad...
Also good are Zool 2 (much better than Zool), Super Methane Bros and of course Bombjack (arcade only, Amiga version is utter shite). (Dunno if Gryzor counts as a platformer, one of my favourite C64 run/jump/kill-em-ups).