Interloper for me. It never gets old.
Definitely Lamda Core
I liked Office Complex, We’ve Got Hostiles, and Blast Pit.
My Favorite and what i think is the best would be Office Complex! Blast Pit is great aswell!
I don’t know about you guys, but I liked Residue Processing a lot. After quite a bit of action, you get a break while everyone thinks you’re dead. And the chapter gives you the idea that perhaps Black Mesa isn’t quite what you thought it was and that Black Mesa is doing some pretty nasty shit. Then you get into Questionable Ethics and you realize just how far that goes.
To me, that chapter is what set Half-Life apart from the other shooter games at the times that were basically nothing but “shoot the enemies”. It was a story element. Yes, today, that is pretty standard but in HL’s time, it was stunning to me and put a smile on my face. 
he propably meant climactic
also, these are my favorite parts:
HL: Blast Pit, Lambda Core and Questionable Ethics, in that order
Op4: Everything after you get the shockroach, except for the parts where you fight those f#!$%§&cking Pit Drones (those are really annoying on hard difficulty)
Blue Shift: The intro tram ride because you can see things like the laundry rooms and the restaurant.
Decay: either Surface Call or Resonance, not sure which.
HL2: tough choice, I think I’m going to go with Point Insertion, with Nova Prospekt as a close second.
Blast Pit. It’s just that one thing that when I hear Half-Life I think of that. As for HL2, We don’t go to Ravenholm was epic.
B-bu-but the chapter after that one was so awesome, with the music from the E3 videos and stuff…
The whole game was epic so whatever
Office complex because of the hallway fights were awesome.
Nihilanth, because the battle is so intense (mostly, when you’re not behind a rock) and the feeling of victory you get after beating it is just undescribable.
Blast Pit and Lambda Core, don’t know which one I like more.
I’m just IN love with ‘On a Rail’ and ‘Interloper’… Not quite sure why…
Surface Tension has about 50% of HL’s best levels- the dam and the cliffside are both in that chapter. I’ve been running through the game again in preparation for Black Mesa’s release.
Same here. I played through all of Half-Life Sierra/WON version run in the Xash3D engine for kicks. I did like Questionable Ethics with all the various rooms and experiments going on there. Lambda Core and Xen is painfully hard with lots of annoying jumping (I missed and fell down so many times). Think I’m getting nightmares with teleporter orbs haunting me.
My favorites are Office Complex, Questionable Ethics and Surface Tension. Office Complex for some reason has a really nice feeling to me, I like the aesthetics and layout of QE and Surface Tension was just so fun to play and look at.
Xen…it is such a freaky world. Me like.
Apprehension, Residue Processing, Questionable Ethics and Surface Tension, I like the second half of On A Rail, well, I guess I just like shooting a lot
We’ve Got Hostiles is my favorite loop, and Surface Tension is my favorite journey.
In episode 2, the part where you’re fighting zombies in the dark and trying to conserve battery power on your flashlight all while trying to get the damn elevator working.