Having been on 19 (or 20 depending on how you count Milky Way) of the currently operating ones, and one that isn't anymore, then I have to say that there are a few that I really love, but also some that are not as fun as the rest of them.
I love:
* Bandit: a really nice ride with loads of speed, just pure fun.
* Megaton: the best ultra-twister I have been on, it's a really fun experience, and the ride-ops are ace.
* Milky way: (the stand up side only) since it gives you the best air-time I have ever experienced, ejector standing up, just an awesome coaster.
Others worth mentioning are: Fujiyama, Mega Coaster, Surf Coaster
There are a few that I don't like as much, but I wouldn't class them as "Death Mashines" since I have been on far worse coasters:
* Zola 7: I mean WTF?? Just rubbish.
* Hurricane - Screw & Loop Coaster: gave us a bit of a beating on one of it's corkscrews, but no where close to what a SLC or Vekoma/Arrow looper does.
* Crazy Mouse (Tobu Zoo): only because my computer didn't like it
Another ride I must add to this list is the Wildstorm flat ride, it's one of the best flat's in existence.