I have to say i thought everything that HL2 tries to do, it does flawlessly(ish). You don't need a completely over the top setting to make a game ooze atmosphere, and you shouldn't need completely over the top weaponry and super powers to make you feel that the game play is interesting. And you shouldn't need to make a character intriguing by simply being unusual/insane or gruesome.
I liked the wasp plasmid, i thought it had a really gritty/almost scientific kind of feel to it.. i can almost imagine such a biological weapon as being possible.. but not the ability to shoot ice and fire and levitate things, that pretty much blew the whole thing into a fairy book for me.
And i like the characters in Bioshock.. but again, only by virtue of them all being very odd.
Half life has to be my choice, and by a country mile.