Ooh, there are just so many to choose from! There's no way I could possibly rate these, so in no particular order:
Slow-mo roll on Hydra: Dorney Park
Hugely overbanked turn/Dive Loop on Cobra: La Ronde
Lie-to-fly/Vertical loop on Batwing: SFA
FSD on Storm Runner: Hersheypark
Norwegian loop on Farenheit: Hersheypark
Giant Cobra roll-type-thing on Joker's Jinx: SFA
High-speed Loop on Rock 'n' Rollercoaster: Disney World
and, for the novelty of it:
Interlocking loops (only ones in the world!) on Loch Ness Monster: BGE