I'm partial to the ability of Pathfinder characters to take both Steamsuit Pilot and Mad Shootist in order to play Samus, in power armor with a sweet blaster. That might be a bit too sci-fi, though.
Mechanically, I like Polyhistor best, because it let me adapt "Elements of Magic" style 'build-your-own attack' rules to martial combat.
Flavor-wise, I love Monuments of War using Post Traumatic Stress as an offensive power.
Narratively, I'm dreading what I have to do to skyseers and Applied Astronomists in the last four adventures.