Favorite Spell: Do people really have favorite spells?
The first spell I thought of for this answer is Imperio, and I'm a little worried what that might say about me.
What I mean by choosing Imperio is not that I think it's the best spell, or that I would go around casting it if I could, but that it's a really interesting spell to exist in the world of HP. Humans have been trying forever to find ways to control each other, through fear, love, bribery, blackmail, guilt, honor, whatever they can. So of course wizards would make a spell that does it for them.
Terrifying.
I also think Imperio is a neat spell because it's the only one of the Unforgivable Curses that can (usually, unless you're Harry) be thrown off by the victim, and possibly one of the only curses of any kind that can be. This says a lot about how Rowling views people, and speaks to one of the main themes of the books: You can kill people, you can hurt people, but you can't permanently take away free will. The ability to choose is extremely important, and the choices people make are far more important than their parentage or their talents.
P.S. While looking at a list of HP spells, I just realized that Peskipiksi Pesternomi is "Pesky pixie, pester no me!"