If only...
So far, my favorite assemblies that I've made, not for most usefulness/cost, but for sheer awesomeness were a F-Mod Biggest Fucking Nuclear Gun in Ao100, and a F-Mod Demolition Minigun that I made in an Ammochain game.
Seriously though, my top assemblies by far are the Tac Boots (149 times), then the Hyperblaster (39 times), and then the Tac Armor (36 times). I don't tend to mess around with other armor assemblies, and as far as boots go, I only ever make Tac Boots or Gravs. For weapon assemblies I use the Hyperblaster for rapidfire games, the Tac Shotty for shotgun games, Speedloaders for pistols, and Piercing Blades for melee games. I rarely make anything else besides those.
But if I could, I'd make a A-mod Tac Phaseshift Armor, and A-mod Tac Grav Phaseshifts. I still wouldn't use any of the heavier armors, I wouldn't need them when I could have that kind of speed.