I'm going to do current top 5.
1.
The Undertaker - He's been using that song or a variation thereof for the longest time, and it's obvious why. It suits him perfectly. It's also instantly recognizable and makes the crowd go crazy. It has a hook at the beginning (the gong) that just gets the crowd to pop. It's an example of a perfect theme.
2.
The Miz. The lyrics, the sound, everything about this song just exemplifies Miz's character. "See all the lights they're glowing/Hear all these people cheer/Feel all the love that's flowing/All just because I'm here" is an incredible line that fits both Miz and heels in general to a T. Combine that with an instantly recognizable intro, and you have a recipe for success.
3.
Daniel Bryan. Yes, I am a mark, why do you ask?

Anyway, at first I wasn't so sure about this theme. It really just shocked me into laughter. And then I realized, that's really Bryan's character. He's a nerd that tricks his opponent into a false sense of security before going out there and tying the guy into knots. The music is perfect for his character, since nerds are supposed to like classical music. It's also unique, just like the man himself.
4.
John Cena. Come on, he sings his own theme and it has the hook. I can't imagine him with any other theme. It's just perfect. When those horns sound, you know it's Cena Time.
5.
Randy Orton. He is a psychopathic character, always on the brink of snapping. His cold fury and mental instability comes across perfectly in this song. I may not always enjoy Orton's ring work, but you can't deny he has a great character and music to reflect it.