Your pick for single best troop upgrade in the game.
I have a couple ideas anyways. And obviously "max troops" are best, but I'm talking about the upgrades that you ran across growing in your career. Obviously hero4->5 is a pretty big one as well.
Giants 5->6. Go from being pretty ok to world destroyers
Loons 2->3. Loons 2 are awful. Loons 3 ain't much better, but are the basis of every easy TH6 3* strategy.
Wizards 4->5. Suddenly you feel like you have some power. They are pretty good for 2 TH levels.
Any other "must have" upgrades you remember on the way up?