Blue Shift spoilers! Beware!
Okay, I guess I disagree with everyone else here - Blue Shift is my favorite one. I see it as a prototype of HL2's character interaction. It introduced an important NPC whom you have extended contact with (Dr. Rosenberg) and some interaction (he boosts you out of the traincar, you help him operate the teleport sequence). I also really like the design of the coolant storage facility, and how it all elegantly loops back to your starting positions.
Yes, it's short, but that also means it's focused: You escape Black Mesa. That's all there is to it. No new enemies / weapons required. It's not an epic messianic journey because it's not supposed to be - after all, you're no Gordon Freeman or Adrian Shephard. You're Barney Calhoun, and you just want to get the hell out of there in one piece.
