Blast Pit. I get a very nice sense of satisfaction at watching the…uh…thing burn. It’s howls of death and pain make me smile every time.
Questionable Ethics.
I don’t know why more people don’t like Interloper. Sure the platforming was kinda weird but the way it showed how the vorts were just slaves to the controllers was well-done.
That said, I’d have to say either Blast Pit or Lambda Core.
I love all the chapters except maybe Chpater 8: On A Rail. Hate those tunnels. Lol, I remember when I first played HL, twas about 2000 I think…spent all afternoon trying to jump upon those darn laser traps in the silo complex, which actually has a pretty neat design inside. But the tunnels, still don’t like them.
Actually, the chapters I like most are all the ones that you run around the offices and labs. I’d love to play a mod, where the Resonance Cascade never happened 
There are many good chapters.
Half-Life:
Unforeseen Consequences - I like the depressing loneliness in those abandoned canalizations and storage facilities right after the incident, the player has no clue what has happened yet (at least in the first playthrough) and is now on his own to find a way out.
Blast Pit - Actually same reason as in “Unforeseen Consequences”, and I like all these damn huge silo, halls, whatever. Yeah, I like big rooms. That’s why I like Portal 2, too.
Questionable Ethics - Nice design of labs, and nice, pleasing atmosphere, especially in the main hall and the laser labs where the sun shines through the ceiling. And good gameplay and combat.
Surface Tension - Great action, and awesome moment at the cliffside. Also, great atmosphere in some places, like the missile storage.
Half-Life 2:
Point Insertion - Great atmosphere, I love City 17. I love to go through all the buildings and running over the rooftops. And that moment when you step out of the station and see the citadel, oh my god. One of the best moments ever.
Water Hazard - Great atmosphere and great gameplay. Much to do in this chapter, much combat but also many physical tasks.
“We don’t go to Ravenholm” - Good atmosphere, many zombies. And of course: fear. A shame that it’s not scary to me anymore, to any of you who played through it several times. When I played the Half-Life 2 demo I almost couldn’t dare going further after I encountered the first fast-zombie.
Highway 17 - Same as in “Water Hazard”, great atmosphere and great gameplay, much to do as well, some awesome parts like going under the bridge and driving over it facing the train.
Half-Life 2: Episode One:
Lowlife - Depressing atmosphere and many zombies.
Urban Flight - Very awesome atmosphere inside City 17 falling apart. Especially right at the beginning when you and Alyx step out of the garage and see the citadel in the distance. Probably the best moment I ever experienced in my whole gaming career.
(Well, I guess I only like Half-Life 2: Episode One that much because of it’s great atmosphere over all. At first I thought it was boring, too, but now I think it’s my favorite part from the Half-Life 2 series. It revives the story and delivers good gameplay with very good atmosphere.)
Half-Life 2: Episode Two:
This Vortal Coil - Good atmosphere, awesome level design, great insight into the life of the antlions.
“We’ve Got Hostiles”-First appearance of the HECU marines, intense combat.
“On A Rail”-Certain segments of the chapter force you to strategize different ways to approach combat, on and off the rail. I find the night-time ambiance on the surface in this chapter to be pretty immersive.
“Forget About Freeman”-I love the feeling of the entire situation going out of control, a great build up before the final chapters.
Least favorite in Half-Life 1 was On a Rail.
Favorite in Half-Life 2 was either We Don’t Go To Ravenholm or Highway 17, least favorite in HL2 was Water Hazard.
Half life- Questionable ethics. All sorts of weird stuff, good story, and interesting gameplay (plus the badass gauss gun part ).
Half life 2- Highway 17. I love the quiet costal atmosphere and the bridge sequence was amazing.
Episode 1- Lowlife, just for how tense and creepy it was.
Episode 2- Riding Shotgun. That chapter is the only time EVER in a video game that i have actually felt like an AI companion was real. That communications tower
part made me actually feel like we were working as a team.
Lambda Core defiantly.
Anomalous Materials rebelliously.
I like Xen.
Tentacle’s singing sounds are the most climatic sound effects of the whole game in my opinion, tied with the horror sounds near red portal leading to Nihilanth. So my fav is Blast Pit, they just coded tentacles perfectly, they’re really climatic and killing them when out of their range is very satisfying. On my 1st ever playthrough I needed to carefully sneak past them, but now I’m able to just run… so it’s not that satisfying anymore ;p
i like that chapter where i do that and fight that.
what do tentacles have to do with climate
I love the chapter in which Gordan Freemanz kills headcrabs! But seriously i like office complex the most!
I have a special connection to Questionable Ethics, mainly because I still remember the good old days when my dad played through the whole game (me watching) and he had to explain what the title of that chapter actually meant.
Half-Life 1
Questionable Ethics
Half-Life 2
We Don’t Go To Ravenholm
Half-Life 2: Episode 1
Lowlife
Half-Life 2: Episode 2
This Vortal Coil
Half-Life: Office Complex
Half-Life 2: Route Kanal
Half-Life 2: Episode One: Either Urban Flight or Exit 17
Half-Life 2: Episode Two: T-Minus One, just the whole thing.
Half-Life:
Blast Pit, it had action. SHOOTING A LOT OF THINGS, and also a lot of puzzles too.
Half-Life: Opposing Force:
Welcome to Black Mesa and We Are Not Alone, those two were the best for me.
Half-Life: Blue Shift:
Never really liked any of the chapters. If I was forced to just choose the best out of them all, it was Duty Calls.
Half-Life 2:
We Don’t Go To Ravenholm, Sandtraps, and Route Kanal.
I haven’t fully finished Episode 1 or 2, but I’m trying to get to them later.
And also here’s my reaction whenever I play HL1/OpFor and See this shark thing.