Imo the best enjoyment to time ratio is:
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Black Mesa (the Xen level at the end was redone and extended compared to in HL1 and it's cool but if it starts to drag just move on)
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HL2 (main game)
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HL2: EP1 start it but maybe skip and watch a playthrough if you get bored
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HL2: EP2
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HL Alyx, if you can play it on a decent headset. DO NOT MOD IT TO PLAY IT WITH MOUSE/KB it will be extremely boring since it's designed for VR
Have fun, there's simply nothing else like Half-Life games
How dare you. Straight to Lemmy jail!