Northwest's domestic used to be crap, but their International service was quite good. Now NW's domestic is getting better, thanks to the Delta influence (at least to my experience).
Delta almost always treats me well, but then I'm a Platinum, and they seem to treat their Elites with a smoother glove than the sandpaper they treat the rest with.
Air NZ have always been great for me. Better than that other Western Pacific airline that starts with "Q".
I kinda liked Czech (last flew in 2001, though).
Korean was okay. Good on some (Seoul-Auckland, Seoul-Narita), touchy on others (Atlanta-Seoul)...
Continental and Alaska are top notch. United has been good, but not great, on both the mainline and Express flights. But my fave US carrier's service is American, by far. Very personal, even in Economy.
The worst, in my experience so far, has been US Airways. I've asked our office booking department to avoid them like the plague.
Best ever in the world, though? Finnair. By a mile... Or a couple Kilometres, at least.
(Having flown about 800k miles so far (500k in just the last 4 years), I have a pretty broad experience...)
(Oh, and if we're talking "hottest," anyone remember Hooters Air? Not the greatest SERVICE, mind you, but awfully fun to ogle...)