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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.
The first is a bit rough. It got a remake with Black Mesa which used to be free, but I don't think it is any more.
I'd honestly just start with 2. It's a better game all round. You can always go back and play Black Mesa later if you like it.
If you have a VR headset and a decent rig, Half Life Alyx is well worth it. I'd definitely play HL2 plus Episodes 1 and 2 before heading into that one.
Sheeiit I absolutely loved the first one, even only having played it for the first time last year. I was blown away at how well it holds up. I do recommend trying it if you have even a morsel of interest. Just my personal 2 cents tho.
Yes, Black Mesa costs money but it's absolutely worth it, they put a LOT of effort into restoring the original as faithful as possible but with more modern graphics (Source engine based). I played the original Half-Life when it came out and Black Mesa definitely gave me the same amount of awe and wonder. One of the best fan remakes of any game I've ever seen.
Also, I see that it's currently on sale for only $5 (75% off), at that price it's a steal and absolutely worth it.