In no particular order, here are my faves
The Brood-Like others said, it was just freakin' awesome and the music was perfect. I always thought Gangrel would look better and add to the feel of the gimmick if he wasn't always grinning like a loon. More sinister look, and maybe a sneer here or there, but his kool-aid grin took away from the creepy/sinister package. Plus, loved his ddt finisher.
Ultimate Warrior-Come on, this was electric. Back in the day, all a crowd had to hear was the first "thump" w/the music hinting in the background and they knew. And they went ape shit.
The Sandman- the epitome of interactive entrances. This was a guy that would own the entire building for as long as it took to get to the ring. He seemed like he took more punishment in the ring than his entrance and crowd reaction would make you believe, which always puzzled me, but the entrance was awesome and will always be one of the best, IMO.
Bret Hart- Like another poster mentioned, the initial "screech" meant you instantly knew what was coming. The confident, slightly cocky stroll to the ring, the giving away of the glasses, and the simple "come on" motions in the ring were nothing extravagant, but he made them, dare I say, excellent.
The Undertaker- I don't think I need to explain this one. Fit the character perfectly and was a show unto itself.
Chris Jericho- Who couldn't feel the anticipation and the electricity of this entrance? Timing was just perfect too, with the new millennium to tie into his "defection" from WCW. It was Jericho's best move, one of Vince's best decisions, and will always remain in a category of it's own.
Randy Savage- while not a super flashy entrance with pyros or lights or pounding rock music, or anything like that, he pulled it off and made it memorable by just being the Macho Man. Plus, nothing says "macho" like coming to the ring and looking like a bad ass to some high school/college graduation ceremony music.
The Boogeyman- Can't believe nobody mentioned him yet. For the short time he was in WWE he always delivered with his entrance. Creepy good fun to be had by all!
And, while not all faves of mine, still awesome entrances that rank w/the best and still had my adrenaline pumping when they happened...
Shawn Michaels/DX- I put these two together because, well, just because. Michaels' entrance was indeed a 'showstopper". You couldn't help but feel the energy and the upbeat nature of this entrance. It was like a pop song you said you didn't care for, yet found yourself tapping a foot, bobbing your head, or even dancing to outright simply because you couldn't help it. DX was the same thing, it electrified the place when you heard that simple, first phrase.
Hulk Hogan- While definitely not a Hogan mark, I will say that I remember the definitive years of the WWF while still a lad. And the Hogan entrance back in the day was just amazing. The music was like Michaels', even if you didn't like the guy coming out to it all that well, you still found yourself singing along and/or mimicking the movements to it. That song, plus Hogan coming out like he was standing up for the world and everything right and good and sometimes even standing up for you personally, was just iconic.
LOD- Come on, the spikes, the face paint, the attitude? These guys just screamed bad ass.
Kane- another perfect fit and awesome entrance. The red lighting, the music, the pyro, all made you feel like you were about to be engulfed by flames.
Mr. Anderson/Kennedy- another case of the entrance matching the man. When he first debuted, I was like "ugh". After a while, I caught myself doing it along w/him. Now, he's perfected it in TNA to the point that you can't help but do it. One of the better modern entrances to match a great personality.
Chris Masters and Goldust- For the same reasons. Very flashy, well produced entrances for the characters and helped garner attention for these guys. This also fits Val Venis.
"Leapin'" Lanny Poffo- Ha! Had to put this one in. Pre-Genius Lanny w/the frisbees and the poems. Could have went in other directions based on Lanny's likes and dislikes, but they matched the entrance w/his interests and was very unique. I liked this back in the day, and I'm sorry, but anybody that can pull off this kind of entrance and actually have a fan following deserves mad props!
And I think I'll wind down the list with 2 more. First off, I want to mention
Koko B. Ware- His gimmick could have bombed w/out the entrance that he had. It was kind of like a flashy, feel good "Apollo Creed in Rocky" kind of entrance. It just made you smile because of how good natured and positive Koko was. And yes, you know it and I know it, Frankie was cool.
And lastly, I'm going to include Kamala. This guy had a great entrance. Basic costume based on his gimmick and actually looked the part and was scary early on as a heel. Kind of got less scary when he became a face, similar to George "the Animal" Steele, but when he was heel he did have a frightening feel to him. On a side note, I was almost trampled by him as a young teen. Went to an event, and long story short, after his match (main event vs. Koko, actually) the crowd was dispersing, but they were still finishing a segment where he chased his handler Kim Chee back to the dressing rooms. I was talking to my friends and not paying attention, saw Kim Chee run by and kind of turned my head to see a big, huge, hulking Ugandan Giant barreling straight at me. I had barely gotten out of the way (I was between the bleachers and the rows of chairs on the floor) before he went flying by me as I thanked the good Lord above I wasn't trampled.