The only reason I'm not a big FPS fan is because they give me terrible motion sickness. That's the only reason you won't find Half-Life 2, or Deus Ex, or System Shock 2 in this list. I've tried them all and had enough of a taste to know they're something extremely special, but I couldn't justify including them without having had enough experience of them.
Halo is one that I was at least able to make my way through without unbearable side-effects. I knew I'd have trouble with it but I was hearing so much good about it that I had to try. In the end I enjoyed it so much it was well worth the times when I did overdo it a bit!
I didn't manage to complete it on Legendary, but I did it on Heroic and that was enough of a challenge to bring out the best in the game for me. I was thrilled that I had to play smart, use cover, flank the enemy, think constantly about my weapon choices. Simply charging into the fray was not an option (if it was I would have grown tired of Halo very quickly). It's packed with exciting set-pieces - the beach landing and later the rock-fuelled exit from the map room, that moment on Assault on the Control Room where you step out onto a snowy battlefield, sneaking up on slumbering grunts and scattering them in panic, running to grab a Banshee as the tune 'On a Pale Horse' kicks in, or the first time you see a three-way battle between robots, Flood and Covenant.... the list goes on.
I love the way the dynamic of the game changed from level to level. A lot of people hate the Library but I didn't find it a chore (though I did manage to get a little lost, mainly because I was by necessity playing it in small chunks). Even backtracking through previous areas was made fresh by new objectives, enemies and equipment. I have particularly fond memories of the section in 'Two Betrayals' where you're faced with a large snowfield, Covenant arrayed at the other end and a selection of weapon options... do I snipe the creatures from a safe distance? Take out the Wraiths with rockets? Sneak around the edge of the field and pick enemies off one by one? It's that kind of moment-to-moment decision-making that nailed the game as a classic and more than just another frantic FPS.
Again, the music is an integral part of the enjoyment. It's epic stuff, punctuated with action-movie moments and signature themes.
Thanks to my poor brain Halo isn't a game that gets much replay, but it made such an impression on me that I'll always consider it among the best gaming experiences I've ever had.