Platformers eh? So hard to chose a favorite.
I think that on the C64 my favorite would be Bubble bobble (This also goes for the arcade version). But seeing as this is an Amiga forum I guess I should at least make some distinct choices for the Amiga. And I didn't like the Amiga conversion.
I really liked the lost vikings on the Amiga, though I'm not completely sure that qualifies as a full platform game since it has a good deal of puzzle elements.
But I really liked Cloud kingdoms, but then there aren't that many platform, but it's still widely concerned with jumping right?
Otherwise Superfrog or Globdule would have to be my favorite. I think Globdule would win over superfrog though.
(And now I know someone is going to come around soon and list a game I hadn't thought of which I liked better)