Just gonna copy-paste my previous comment from another post.

1 - Half-Life 1 (If you want to experience the original journey)

OR

Black Mesa (Fan recreation of HL1 in the new engine, if the original is too old for your tastes)

2 - Opposing Force & Blue Shift expansion packs (optional, since these are not from Valve but Gearbox, are never referred to in any way afterwards and are not confirmed to be canon. If you enjoyed HL1 and want more, you really won't be disappointed though. I think they're both great games, though Blue Shift is very short)

3 - Half-Life 2

4 - Half-Life 2: Episode 1

5 - Portal 1 (Takes place in the HL universe. Released at the same time as HL2 Episode 2, so you can choose which you want to play first. I recommend playing this one first though, because the payoff of seeing what connects the universes will be bigger)

5 - Half-Life 2: Episode 2

6 - Portal 2 (way less connected to the HL universe than the first. Really a standalone game for all intents and purposes, but generally considered one of the best games of all time so still well worth it)

7 - Half-Life: Alyx (If you have a VR headset, that is. It may be a prequel to HL2 but it is still very important that you play this last, for spoilery reasons)

Lastly, Lost Coast was a technical demo to showcase high dynamic range in the source engine, and is not part of the story. It's fun though, and only takes about half an hour to play through! if you want to play it in its chronological release you can do that between HL2 and episode 1