So we have:
Half Life (Talk to me later, I'll buy you a beer)
Half Life 2 (The popular one that gets way too much credit)
Episode 1 (Basically the City 17 battle again, but longer)
Episode 2 (The troll cliffhanger ending that you'd think people would be over by now)
Blue Shift (I guess Barney only figured out how have a personality after the Black Mesa incident)
Opposing Force (Gearbox actually handled a separate license well with this and Blue Shift. How these didn't end up like Colonial Marines or DNF, I will never know)
Black Mesa (The well known fan project to create Half Life in the source engine. It's in my library but I'm not gonna play it till they actually finish the thing)
I personally like Half Life the most. It had great pacing, a good sense of direction, it made a smooth and natural shift from survival horror to tactical FPS, and unlike Half Life 2, you don't have to drive a crappy car for half the game. Seriously, who actually liked the driving levels? Sure it was cool at first, but it just kept going...and going...and going...and going...and going...
I just think HL2 didn't have any real focus. It spent too much time on the wrong mechanics (boy do I love putting heavy objects on one end of a seesaw!) and not enough on real gameplay. Except Ravenholm, that place was fantastic.
Then again, Nihilanth was the most frustrating thing in the whole Half Life series.
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None was bad in my opinion