I think Ghosts N' Goblins on the C64 is better than on the Amiga. Sure, the Amiga version has better graphics, all the levels and more advanced enemies. But there is one thing it doesn't have, and that is the somewhat weird but highly atmospheric music of the C64 version. Ghosts N' Goblins isn't the same without this music imho.
I also prefer the C64 version of Barbarian 2 since I find the Amiga version to be much harder, but this might be because I only have a pirated Amiga version of this game.