An ichthyosaur. They scare the fuck out of me.
Having your brain eaten by an Advisor is probably extremely agonising, perhaps even more so than a headcrab come to think of it.
For me, while the death itself would probably be quick and painless, going toe-to-toe with a Gargantua would be the worst way for me to die, judging by fear value alone.
I’d probably say something launched from the gravity gun. It seems kinda embarrassing to be killed by a physicist shooting a toilet at your skull.
What.
Either their Autocorrect failed to recognize their nasty misspelling, or someone’s got some unsung sexual tension.
Poison headcrabs. Not only you’ll suffer everything a normal zombie goes through, but your body will be painfully manipulated as a sort of home for more parasites, having your flesh bloated and ripped from the neurotoxins they stick in you. Everything while you’re still concious. Why else would they laugh once you kill them?
Also remember the snarks explode if you hit them, so if you swat them away it will be a finger or two less, not counting the chemichal burns.
Who’s to say you have any kind of consciousness once a headcrab’s gotten intimate with your neurons? And it’s in the headcrab’s interests to enact that transition quickly, so the victim doesn’t have time to bash or stab it while it’s sitting stationary on their face. The noises of pain made by fiery zombies may even be the headcrab’s, transmitted via the host’s vocal cords.
Death by barnacle could be one of the worst, depending on its exact methodology. Assuming it’s exactly as experienced by Gordon - gradually chomped within a giant maw, rather than a quick tongue-induced neck snap - it’d be like being chewed in half by a giant great white or tiger shark, rather a horrible way to go. If you’ve seen Jaws 3, you may have some idea of what that’d be like.
Snarks… ouch. Grunt hornets… ouch. Don’t underestimate the Garg either, being burnt to death is WAY up there, even if it just took a couple of seconds.
Personally, though? I’ll vouch for masturbating in the Anomalous Materials hot tub (most of the scientists probably did; groundbreaking work to advance humanity would make you super horny) during the resonance cascade, then having a leech swim up and infiltrate one’s exposed nether regions. I assume it would then slowly electrocute you to death from the very worst place imaginable.
Elecrocute? What the fuck are you talking about?
Do they not electrocute you? Their ambient noise in HL sounds rather voltaic, and they make an unreasonably loud noise when they hit you. Maybe it’s just my imagination.
:edit: Apparently it is. Huh. I’ve spent 14 years assuming the little buggers were shocking me.
I’m kinda surprised no one has said, “Walking over a physics object.”
For me though, it would be the barnacle… while on the way up, you’re looking right into it’s maw while thinking, “damn it, how the hell did I let it get me?”
I would hate to be climbing up a ladder, then all of a sudden not be able to get off, or I get off, jump, and die. It would suck.
crushed by the garbage compactor?
Doors, man. Fuckin’ doors.
Not to mention lasers, electricity, and random portal energy beams of death.
And ELECTRIFIED TOXIC WASTE…
The bullsquid. I imagine myself laying in the ground, with my skin covered of that acid spit and crying in pain, while that monster calmly comes nearer, and starts eating my legs, then my torso. It would be the most horrible thing.
The fish… Ichthyosaurs. I fucking hate sharks and that would be a horrible way to die.
Another way would be the tentacles, barnacles, or a hunter.
a french kiss with an headcrab 
even though, in intimacy, I prefer to call them:
1- headcrap
2- facehuggers (copyright freeman’s mind)
lol!
Freeman’s mind is hardly the first place they’ve been compared to facehuggers.
I’m rather fond of calling them headhumpers, myself.