If you're talking about right now, at this moment, then call me crazy, but I'd say Borderlands. It's insanely fun with great characters and excellent writing while at the same time scratching my loot whoring itch. If I could only have one FPS going forward, it would be Borderlands without a second thought.

This.

Battlefield and Counter-Strike options are missing from the poll. I have to vote for Half-Life, the second is one of my absolute favorite games.

Pokemon

Halo, it invented or revolutionized so many things, way more than any of these series and more than almost every series in gaming.

Halo for sure. I didn't enjoy the genre until i played it. After one go i loved the game.

Like what, for example?  A quick run-down of the major points would be nice.

6th gen. Black
7th gen. Starhawk/KZ2
5th gen. Goldeneye 007
PC Doom
3rd gen. Duckhunt

in the end i'd have to go with Doom.

Overall Halo. The gameplay is consistently solid, but that's not why it's my pick.

ATMOSPHERE, ATMOSPHERE, ATMOSPHERE. Halo has possibly the best orchestral score of any FPS or any game for that matter. The story has never been amazing, but the atmosphere is the best. That epic feeling of the sides of the screen getting black and walking into a large, open area with a huge draw distance in the background is second to none.

The online is a great experience. Many great moments playing Halo 2 with friends, Halo 3/Reach online, and although I've yet to play H4 (No 360 anymore) I'm sure it'd have a lot of great moments.
It's really the sum of it's parts. Put everything together and you have the most complete FPS series ever.

Timesplitters. Oh sure some of the other games may have made bold new steps in gameplay innovation. But Timespliiters was the only one that had a playable Ninja Monkey character. And for that reason, it wins.