I think that if you want to go into the ring, you want to make a statement. It can be that you're mean and aggressive, or confident and unafraid of fighting. Personally I think aggression is best song wise, and what better much to use than thrash metal, the extreme subgenre of heavy metal.

My first suggestion would be Holy Wars...The Punishment was Due by Megadeth

http://www.youtube.com/watch?v=-YczYC2yHTM

My second choice would be Metallica

http://www.youtube.com/watch?v=NUQz_eTe4kg

If you want to go power metal, I'd suggest Angra's Temple of Hate:

http://www.youtube.com/watch?v=Bo9yoHXVtME

And if hard rock is your thing, Disturbed's Indestructible is a good choice

http://www.youtube.com/watch?v=BWRjZiXILDM

I don't like Judas Priest, Venom, Slayer, or whatever else. But I tell you, classic thrash metal is aggressive and will probably scare your opponent a little bit.