I'd play HL1, OpFor *and* Blue Shift.
HL1 is, well, HL1, plus it's good. Also, HL1 has *the* Glorious Gauss Gun.
OpFor is pretty great, but it tapers off towards the end and the ending is meh, feeling like the entire thing has been sidetracked.
BS is the weakest (mostly due to having *less* stuff than the original), but is helped by remaining consistent to the end. Plus, BS has Xen sections that aren't durpy platformers.
Both expansions are nice in how they intertwine with the original.
Then move to HL2.
(you can't be a graphics whore, HL1 did not age gracefully).
I'd say it did. Not as well as Unreal (obviously), but it looks pretty solid.
Its main weaknesses are blurry textures on scenery and weak lighting model.
Other than that it's only really held back by being mostly set in rather uninteresting interiors, but that too changes towards the end.
HL2, of course, aged *very* gracefully in terms of visuals.
Also one thing that is often forgotten when countering praise of HL's AI is that creatures in HL use different senses and have fairly diverse behaviour.
It isn't just how good the AI is at killing you, it's also how interesting and convincing its behaviour is.
HL AI is nor just mehreens throwing grenades at you when you camp behind a box.