I'll make this a really simple format - just list 'em, for Pete's sake! In the order that they come to me...
Truly Cool:
Nosferatu - The best thing about these guys is that it's so much fun to say "Nosferatu." They were the best unit in the Transylvania scenario in Warlords II, my first introduction to these guys. When I was talking to a soldier at work about Vampire, he seemed pretty enthusiastic about these guys, and I gotta admit that I am too. The whole idea of being hideous combined with their ability to remain unseen is way cool. Plus, they hate a lot of the clans that I do.
Malkavians - Like the Nosferatu, these guys don't really fit into vampire society, and thank goodness for that, since vampire society sucks. It's cool to be crazy, but even better than that, everybody thinks that these guys are dangerous. Oracular abilities are an easy way to get in good with the rest of the coterie. Did I mention that they were all crazy? Oh yes I did. So now I mentioned it twice. Why's that cool? Well it helps when you have to suck peoples' blood and fight ancient evils. Who cares about angst when you can just slip into a different personality and let HIM (or HER) deal with it.
Sorta Cool:
Gangrel - It's cool to have a close kinship with animals, but come on, these are vampires you're playing, not werewolves! Pick a different game, jeez. I don't get why these guys are closer to their Beast than anyone else. Shouldn't that be one of the two above? But ah well. Yes, Gangrel are cool.
Ravnos - Gypsy chicks can be hot, and it's way cool to be the master of illusion. Just remember how cool Gambit was in the X-Men to know how cool the Ravnos are. Downside? Not Camarilla. Darn.
Brujah - It's fun to be an unthinking meathead and just blow up everything that pisses you off. Unfortunately, every other Brujah player in the world is going to be doing the exact same thing. Make a Brujah with depth? Be my guest... tell me how it works out...
Tzimisce - Actually rather cool, if they wouldn't be Sabbat. But nobody really wants to play Sabbat, at least not me. If they'd be indepedent, they wouldn't be on this list.
Not Cool: aka The Rest
Toreador - "These pusbags sure make themselves easy to hate, don't they?" says the Nosferatu page in the rulebook. I still can't think of a single TRULY good thing about being a Toreador.
Ventrue - Vampire aristocrats? Fine, if you happen to like politics. So unless your last name is Bush or Byrnes, you probably won't want to play these guys.
Assamites - A great excuse to make Fatima into a character to get teenage guys into Vampire. Other than that these guys are almost impossible to play in the actual game, and are one-sided otherwise. Just look at the Path of Blood to know how annoying it would be to play one.
Giovanni - If I bought Wraith: the Oblivion I might be tempted to play one. Oh, wait a second, I meant I'd be tempted to play a WRAITH. Who'd want to be Giovanni then?
Lasombra - First of all they're Sabbat, which makes them bad enough, but they're as close to vanilla Sabbat as you can really get. Ugh.
Tremere - [Time out: by now you mighta realized that I switched to alphabetical order, courtesy the Brujah page in the rulebook. Ok, time in] If I bought Mage: the Ascension I might be tempted to play one. Crap, deja vu. Moving right along...
Children of Set - [Forgot about these guys; I'll add 'em at the end] Nobody likes a Setite, whether a vampire or someone playing a vampire. I mean, they basically have "don't trust us" written on their foreheads: no player would seriously like these guys, would they?
Ok, I'm done whining. Now what do you think?