"Best Amiga mods" really depends on what kind of music you like. I've downloaded many "Top 10" mods only to be disappointed because they were rap or guitar crap.
My favourite (non-game) mods are:
BubbleDreams
Desert Dream (mod 2)
Desert Dream (mod 1 - edited)
Lethal Exit
Das Boot
Flowing Sky (edited)
... and bits and pieces of mods from other demos (where I only like one or two blocks from the mod). If I add game mods (mod format only), I'd add:
Lotus 2 Title (edited)
Beast 2 Title
Beast 2 (starting level)
Beast 3 (esp. level 3)
Agony Intro (and level intros)
The Settlers
Stardust Title
Zool 2 (starting level)
Fantastic Voyage (edited)
(of course there are lots more awesome game music, but they're not in mod format)
