I'm not one for FPS's, but i did enjoy Duke Nukem 3D and Battlefield Vietnam.
To anyone who says Halo, i feel that series is the worst thing to ever happen to gaming, simply because the standard it has set held gaming back for many years. A graphic system that, even for it's time, wasn't the greatest (Dues Ex with more detailed textures, in my opinion), gameplay that, yes, set the standard, but can't be considered INVIGORATING (Again, i compare to Dues Ex, which I find new ways to beat every time i play it), and a story line that...well...I've watched porn with better story line. At least something is pleasured with porn, and i can't say the same with Halo.
I've had a run in with Brink, and i'm quickly falling in love with it. Recommend, but it will take time for me to think of it as the best FPS of all time, if it is worthy of that title.