HL2:Episode 1+2 are continuations of the story after the events of HL2. It took me forever to realize that HL1:E1 wasn't the first half of HL2 lmao, they're actually sequels. They should have called them "HL3: Ep.1/Ep.2" to be less confusing imo.

Deathmatch: Classic is Quake's multiplayer in GoldSrc, and Deathmatch: Source is HL1's multiplayer but in the Source engine. They're not worth bothering with tbh. The Quake remaster and the original HL1's multiplayers are way better than those games, I highly recommend them if you're into chaotic arena shooters. (Only if you're willing to get your ass kicked while learning, though ;P)

Tbh Opposing Force and Blue Shift aren't the greatest, but they feel like very high effort mods which is about as much as I could as for. I recommend trying them but I wouldn't feel too pressured to stick it out all the way through if you don't feel like it. Their stories are trivial, and the OF protagonist never even comes back. But HL2:E1+E2 are pretty good, albeit flawed compared to HL2. Highly recommend them in their entirety. HL:Alyx is also great for what it is, a AAA VR experience. I highly recommend that one too.