Hulk Hogan - It didn't matter if the crowd just sat through 15 matches (most of which were garbage), when the start of Real American hit, the fans STILL managed to have enough energy to go bonkers for the man (I am, of course, referencing WM6). between 1984-1990, nobody got (and still don't get) the reactions Hogan got every time his music hit.
Undertaker - There is absolutely nothing more iconic than the sound of the gong. The attention this man gets during his entrance is surreal. One memorable example was when I was at SummerSlam 2004. During the Diva Dodgeball, pretty much everyone went for a smoke, to buy conessions or to the bathroom. But, when Taker's gong hit, everyone abruptly stopped what they were doing and went back to their seats. A personal favorite entrance of Taker's was WrestleMania XV. Taker wore the sickest outfit I have ever seen.
Kane - I don't think I have ever heard pyro quite as loud as Kane's. The stage pyro is loud enough but the ringpost pyro is absolutely ridiculous.
Chris Jericho - Like Kane's, Jericho's pyro was ridiculously loud c. 2008. But the combination of the countdown, the lights going out, the pyro, the music, and the pose make this one of the greatest entrances ever.
Gangrel/Brood - Same reasons as Ezekyo... Gangrel rising up through a ring of fire was spectacular combined with the eerie lighting and music. Sadly, this awesome entrance was wasted on someone who quickly became irrelevant in the WWE.
Boogeyman - Yet another wasted, kick-ass entrance. His wall of flames pyro was probably better than Kane's and his crawl through the pits of Hell was pretty cool too.
Triple H - Specifically, the time he was using My Time as his theme. The dark arena, the intense, multi-colored strobe lights... this entrance looked good on TV, but was off the charts in person.
Demolition - The greatest wrestling theme I have ever heard. That, and when Demolition got to the ring and removed those sick, black hockey masks to reveal multi-colored painted faces and red tongues, that was kick ass.