Uridium, in fact most ported shooters. Uridium 2 is very good-looking on Amiga, and Braybrook did a great job - except I somehow just don't like it. 25fps makes it even easier to fly into enemies, the v-scrolling thingy confuses, and gfx are nicer but less distinct.
In fact, shooters that are <50 fps are all quite misconceived in my book. They are the easiest genre/game idea of all to adapt so that it will run 50 fps, and it shouldn't really be a problem.
Most conversions of early (as in released first on 8-bit) were low budget, so it should be easy to find lots.
I always thought Graftgold did a proper job converting their games to Amiga, so I must strongly object to the C64 version being better! I spent hours and hours slodging through the second ship, I felt quite at home on the spaceships - especially with the great job they did on the sound in Paradroid 90.
I'd agree with Power Drift, but I think IK+ is utterly perfect on Amiga - perhaps the C64 one also is. Ghosts'n'Goblins on C64 is of course the ultimate version on any home computer, even though playing it now is too hard - much harder than the arcade version. Giana... well, I think it's excellent on Amiga. Lot of the graphics look crap, but it's playable, easy, lovely.
Thought Cybernoid I & II were pretty shallow (low lastability (c) Zzap64), C64 had the masterful music so it wins, easy.
Commando rocked on C64. Ikari C64 wins easily, Amiga version was worse than the Atari ST version... Rambo III, C64 wins again...