There's a nice article at Games Radar giving their take on the 101 greatest
gaming moments in video games.
Here


They mention moments from both Half Life & Half Life 2, but if I were to think
of unforgettable moments from either game, it would be their uncanny ability
to elicit feelings of vertigo from me. I'm not one to get motion sickness or
have any other kinds of physical reactions while playing games, but leaping
across clumps of rocks in space in Half Life or picking my way across wind swept
bridge girders in Half Life 2 had me genuinely afraid to look down! I never even
knew I was afraid of heights until I played those games.