I have been playing the level editor a lot. Here are some of my favorites:
Contract (Hitman themed, start with silenced pistol, custom 47 player model
Hard Rock (Jacket, night club, lots of enemies, dense areas)
Moonlight (reminds me of Deus Ex, Hammer, very nice color scheme)
Strike (Jones gator mask from 1, bowling alley, start with fire axe)
I haven't played any of the custom campaigns yet, but I have a lot installed. I have been mostly doing hard mode so I want to focus on that, but some of the levels that people make are really cool. I even have a custom campaign with multiple endings. Really cool stuff.