Half life has some of the best AI in gaming yet! The marines and the stealth ops especially, were dope AF flanking and bombing your ass out of cover.
I always felt harder difficulty in half life required you to think more and be more tactical, unlike newer games which just increase enemy stats and decrease yours.
Plus, Half life had two of the coolest expansions ever!
HALF LIFE and BLOODBORNE GOAT CLUB
And then there was those ninja bitches. The first time I encountered them back in 1997 and those damn super quick footstep sounds...dayum!