A very formative game for me, impossible for me to be objective here. For about 3 years of my childhood, at around age 10-13, on any given day I was at some point in a long, continuous chain of Half Life play throughs. I would finish the game and immediately start up a new one. Sometimes I'd even start a new game before I'd finished my last one.
Sure, it has its flaws. It hasn't aged especially gracefully, the little platforming puzzles aren't the greatest. The last half of the game can really drag. But I could have done a lot worse for an introduction to grown up games.