I got Half Life when it came out in the 90's, it was a complete game changer for everything. The way it went through the chapters from the lab to Xen and beyond was nothing I'd ever seen before it just totally redefined gaming. Even the opening scene where you went through Black Mesa on the monorail passing places (you didn't know you'd visit later in the game) was a masterpiece - though in the first playthrough you wouldn't know you were going to be there, subsequent playthroughs you'd chuckle to yourself when you went on the monorail journey.Lately I've been playing a bunch of Half Life and Portal. I've played either series enough that I'm looking at the commentary now.
Voice acting further immersed you in the game, the actor playing G-Man was peerless, the broken inconsistency of 'his' speech further added to the sense of foreboding. I still don't know whether he was the good guy or the bad guy.
Half life 2 gave us the Vornigaunts having the collars round their necks controlling them by turning them from enemies to friends when they found a way to break free, on a couple of occasions they saved the lives of Gordon and his friends, another nice touch in the HL lore I reckon.
Portal - What an excellent game, so much humour with the autoturrets getting so depressed when they can't kill you.
Never to this day will I understand why the mythical Half Life 3 wasn't developed, unless they were worried it would become a franchise that would lose quality as we've seen with so many other games.