Doom something, probably the first one, or maybe second. The next is probably golden eye, where I only played multiplayer with friends, since I didn't have golden eye, and where I spent most of my time rushing them with my slaps while they wielded AK-47, or pencils as I called them. Then some tries here and there. First that I played a lot was Timesplitters2 and the first that hooked me a lot was TF2.
That's more then just the first one, but it's about the first of each of their degree of playing, more or less. Official answer is still Doom though, in a dark room.