I like the Trauma-only Forest level the best. The music is great, and the fights are fantastic, even if they're a little short.

Nothing can beat Hell for how cool it looks, though.

hard to choose...but probably the Cathedral :)

favorite lvl? this is difficult because they were all awesome. i like all chapter 3 lvls ( station, factory, base), but i also loved the arabic lvls. but hell's design is unmached. a gigantic battlefield which froze the second a nuke exploded? that is just genius. lucifer was no fun, but the map was a pearl.

Snow Bridge because it is too hard to find all secrets :)

Old Monastery, absolutely! Beatiful scenery, wonderful ambient music and exactly type of level which I love...

My favourite would be Palace. It has great design, some of the more challenging secrets and in my opinion, best ambient music. I propably like it so much because it was one of the 3 levels included in the demo.
I have to admit both City on the Water and Docks are very good levels because of their secrets, those secret areas are propably the hardest to get (there is also this one secret in Catacombs that's very annoying, but at least you're not bunny hopping on a huge crane).
Fav boss battle has to be with Alastor, it's the most challenging one.
And from the expansion, which I haven't finished yet, I really liked Lab for it's combat music and the introduction of Bolt Gun (and a few annoying tries for the card because i run out of ammo, but it only adds to the challenge). Also, Stalingrad was really well-designed.

It was a neat idea, but I'd like to see who could shoot all the creatures at that speed. It could have been a bit faster especially as it runs so well on modern machines. I think it added to the game and was fun.
It was a brilliant design, just a joy to play through this one. You're like, surrounded by freakish ghouls and then you see more of them crawling across the ceiling. And they created some tense close ups with those electrified dudes that were actually rather frightening! :)

What is the story though?

I was trying to figure out what those zombies were throwing. To me, it looked like they were throwing a hellishly spawned fetus. Where do they take it from? Very strange, and this level is as grim and ghoulish as you'd expect from a good game.

The fight at the end by the altar was a nice touch... as were the flying witches.

It's really difficult to choose a single level... they're all so good! My favorite level, all things considered, is probably "Town" from chapter two, in terms of atmosphere, design, gameplay (especially if you accept the challenge to complete it using stakes only). It's also the first level I've ever played (in the demo), so maybe there's a bit of emotional attachment too (I was immediately hooked).
But I have to mention "Military Base" as well, for its sheer magnitude and the epic scale of its battles.
Other levels are great, though. "Hell", for example, is a stroke of genius and a work of art in conception and design, even if it's a bit disappointing in terms of gameplay (I found the final battle with Lucifer to be sort of anticlimatic).
"Old Monastery" is another wonderful level, with tons of atmosphere and an intricate design.
As far as atmosphere is concerned, let's not forget the "Asylum": short but sweet, it's the creepiest of them all.
"City on Water" is great too (shooting demons in Venice! Who would have imagined?)
"Snowy Bridge" is another level that scores high for originality, execution, complexity and size.
Finally, I would like to mention the "oriental" levels, "Palace" and "Babel" for their sheer beauty - the latter in particular, clearly inspired by Brueghel's painting, which wallows in a dream-like, fairy-tale aura.
Oh, and the first encounter with the Necrogiant in the "Enclave" is unforgettable...

(note: I haven't played the expansion yet)

You're probably used to newbie games.

Loony park is one of my favourites too, but I played the HD version of the map as I hadn't played BOOH just then. the Coaster section Is like the rocket ziplines in Circus Park in Shadow the hedgehog but with demons instead of mechs, and sadly no soaping ropes nor fun extra carnival games and it also has the feel of the arcade classic CarnEvil.

Snowy bridge is one of my other top favourites (I say top favourites because most levels have an appeal one way or another.) because of that section where you walk up the gondola cable.

You're supposed to ride it but it doesn't trigger unless you stand on the car and it's an arse to stay on without it jiggling about under your weight and throwing you off. Not to mention its more exhilarating to walk up the cable and easier too.

The HD version forces you to ride the car but without the collision problems of the original and another one is added. you also get to go through an added daytime section of the ghost town at the end.

I haven't seen all the levels, mainly ones on higher difficulties only because Painkiller has the Doom and Halo 2 problem of the highest difficulties being completely unplayable, especially in Hell and Damnation.

Catacombs and Town, both from the original Painkiller. They have such a lovely, dreadful atmosphere to them.
'specially the Town.

Overdose tried to replicate the Town, but instead only made a small village. It work just as good.
I wish every other game after Overdose didn't suck so bad.

The monastery level...