Bret Hart - Either the original or the 1994-97 version. I don't care for how they rearranged it for the new one. This music is synonymous with Bret Hart, and always will be. It's part of why it never felt right when he came out to the ring each time in WCW. It's a relatively simple piece of music, but it totally embodies the spirit of the Hitman and all that he stands for. Perseverance, never giving up, being true to yourself, doing the impossible, and always believing that "even the biggest dreams can still come true". Growing up and hearing that music, I found it incredibly inspiring, and just having it in my head during tough times made me feel like I could get through them. The way he hobbled to his feet, barely able to stand and raising the belt above his head after he beat Yokozuna at Wrestlemania X, it just would never have felt the same or had such an impact if they were playing different music.
Stone Cold - From the moment you heard the glass break, you knew what was coming, you knew who was coming, and yet you still didn't know what he was going to do and couldn't wait to find out. They have often done the initial noise/chord/quote at the start of a theme to gain attention, but the shattered glass brought people to their feet like nothing else, with the possible exception of Undertaker's bell. The bad ass quality to the song itself suited his character quite well, face or heel.
Undertaker - I remember him coming out to his initial "Rest in Peace" theme when I was young and it was kind of chilling. He set the stage for a dramatic entrance in wrestling, and I don't think anyone can argue with that. The current, remixed version works pretty well, with the electric guitar adding a bit of an edge to go with his more modern take on the Deadman. The Ministry era theme was absolutely epic, and had this great sense of foreboding and evil to it. The choir, brass, organ, violins, repeated tolling of the bell and his speaking in tongues over the music combined with the crunchy electric guitar riff created an unparalleled sense of drama that has yet to be equaled in a wrestling theme. The bell has always been the most recognizable sound in wrestling music, to the point where they incorporated it into his biker music. As for that, fuck the entire gimmick. I can't think of any worse way to destroy the mystique of a character.
Mr. Perfect - There are few words that are really needed. This music was epic, and always made me think of conquest and military battles in ancient Rome. Coupled with a character as bombastic as Mr. Perfect was, and the fantastic, superior quality of the matches that Hennig put on between the music, few people were ever as deserving of such treatment, and even fewer could have made their presence while it played believable or justified.
Hulk Hogan - This song, as much as I hate when it (still) gets stuck in my head, embodied the spirit of what the Hulkster preached, as well as what he did in the ring. He was all about American values, saying your prayers, taking your vitamins and always coming out on top, even when the notion of doing so was just plain silly. He was really an embodiment of that, in an almost Christ-like cultural phenomenon (even I took Hulk Hogan vitamins), and his music hitting made kids jump out of their seats. Hulk was practically everyone's idol, at least for a short time, and his music went along with him and his character perfectly. This is all why him going to WCW, turning heel, wearing black and coming out with different music was so ridiculously shocking at the time. When this music stopped playing, it was like the Hulkster and Hulkamania died with it.
Honorable mentions: Demolition, LOD, HBK, Goldust, Razor Ramon, "The Model" Rick Martel, and The Mountie (yes, damn right I went there).