Being constrained to riding only the UK's major coasters thanks to a lack of money, I would say the two best drops on any coasters in the UK are Oblivion's monster of a drop, thanks to the immaculate buildup of tension prior, or Megafobia, thanks to the all out ejector madness (if you're riding on the back row).
If drop towers are included though, that would have to go to the stand up side of Apocalypse at Drayton Manor, there's nothing else quite like it that I have ridden. That drop is just incredible! Shame it's not at a better park in all honesty...