It's kind of tough stuck between Curse, Outrage, and Leaf Blade.
Leaf Blade has a beautiful animation and I've grown so fond of it since its power upgrade in Generation IV. Power of 90 and a high critical-hit ratio is not to be underestimated, especially when you have 15 opportunities to use it.
Outrage is just so packed with raw power. It's the most powerful move that doesn't have a drawback as far as loss of turns/recoil/fainting. In Generation V I'll also like its equivalents Petal Dance and Thrash, but not as they remain at a power of only 90.
Last but not least, Curse. I love the increase of both Attack and Defense and I love the way you can strategically use the loss of Speed (i.e. Trick Room, Payback). It's also so nice on Ghosts and I love having it on my Umbreon. :D My favourite stat-changing move.
However the thing that's true blasphemy is changing the type of Curse in Generation V. ??? is the best type ever. :(