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Half-Life 2 is currently 100% off for its 20th anniversary, plus a major update
(store.steampowered.com)
Bonus: it also seems that the episodes have been rolled into the base game. Full details of the anniversary update.
Personally, I think 1 holds up better. 2 isn't bad by any means, but it was one of the first games with physics and the physics puzzles get old fast. It was amazing at the time, but now it's not as interesting because we've seen it a million times.
1 is just a multilevel death match.