I have to say I would add this to my favorite moments. This speech deserve a Nobel Prize!
It was definately spine-chilling.
Gman at the end
Water Hazard. I LOVE Water Hazard.
But I’m kinda torn between this and:
The Barn Advisor
The Fast zombie jumping on the ep2 car
Gman’s ep2 speech
Eli’s death (that was just so… depressing)
EDIT: So, yeah. I liked ep2 the best, as you can probably tell.
Oh shit! I forgot about that fast zombie jumping onto the car. The was totally epic. But I am torn as well since Eli died at the end
After Ravenholm when Winston gets shot and it plays 60 seconds of "Triage at Dawn, also Gman’s monologue at the end of HL2 and in ep2
The first part of E2 and your in the building and alyx is outside. Lights turn on and you see the highlight of a hunter, and a dead rebel hanging by his foot. You keep going until alyx gets attacked, and you freak out.
HL1: Office complex, when it creeped me out so much I could only play like 10 minutes a day.
Opposing Force: The Biodome Complex, seeing all the stuff that happens after Freeman leaves Earth. Also, Crush Depth, the feeling of claustrophobia and the entire level design was great.
HL2: The Airboat: when you were free to go to so many places, and the awesome graphics. I loved when you went into that tower thing to open a gate, and you had so many options to kill the Combine. I loved when the metro-cops die and they make that beeping sound with the woman voice announcing their death.
EP1: The entire initial Citadel part, I loved how it was falling apart and it reminded me a lot of HL1 when Black Mesa was falling apart. I love places getting destroyed while you’re in, with half-working equipment and explosions.
EP2: The amazing graphics everywhere, especially the realistic goo in the Ant Lion nests.
The entire car ride in H-L2 Highway 17, they should’ve added more explorable places though, and less loading times 
Thankfully, the loading screens were in nondescript tunnels and not out in the middle of the field.
HL1: Blast Pit
HL2/episodes: that place where that frog stabs Alyx.
frog??? WTF?
:hmph:
Favorite Half-Life moment? That would have to be when I went to seal the Silo 2 Doors. It surprised me when the rebel goes “Hey! We could use an extra- AGH! RUN! RUN FOR YOUR LIFE!” when the hunter surprise attacks him. I almost tried to run under the door, then realized I couldn’t. Then when you see the Advisor at the top.
I lost.
So did I. So did everyone.
ribbit
It’s Imonfire!
Uplink: “maybe he can do it.”
HL1: shooting my first scientist and killing Barney with the crowbar.
HL2:When a Zombie screamed, “YABBA MY ICING!” and killing the crazy guy who paces at the beginning with impulse 101.