Of the ones I've done, Zadra hands down. Ben is right, it's the only one which actually feels like those scenes in the Matrix where time just seems to stop, bullet time I think it was called. No hang time whatsoever, pure zero gravity, as the element is supposed to deliver.
Worth noting I haven't done a couple of the notable ones, like Aireforce or Batman yet, so they may well be as good or better.
The ones I have done like Velocicoaster, Gwazi, Hyperia etc, don't really come close. And I can only assume those voting Voltron's stall haven't done any of the big stalls, as it's tiny and forgettable. It doesn't deliver a zero g / weightless feeling of a 'zero g stall' at all.