Depending on where you live (because online playercount is important) Battlefield 1942 is still the one to beat. Not because it did it all first or nostalgia or any of that crap- but because it still has more variety, over a wider range of places, with better balanced weapons and vehicles than anything else, and that includes all the other Battlefield titles. Not to mention the mods available give it a huge amount of replayability.
It's also free.
If you can deal with the graphics, you'll find a rock solid shooter (it has to be, considering people still play it all these years later) that puts more units on screen at once with more player control than modern BF would know what to do with- given the series has long since abandoned the scale of fights in favour of shiny graphics for some reason. You won't see a modern Battlefield, for instance, ever come close to the size of
this ocean battle that someone recorded not two days ago:
Just remember-
everything you see in these videos can be driven by a player.
Alternatively, if you want something
completely different, try Mobile Forces. This goofy game from 2001 had many of the staple FPS match types from Unreal Tournament, only with a range of ground vehicles with the most entertaining physics around. It's a
very budget game and the production values show. And yet... there's something oddly compelling about the gameplay that keeps you coming back. My personal favourite mode is the one where there's a bomb on a car trailer, and you gotta drive it into the enemy's base to set it off before they drive it into yours. Can be played online or against the bots, which do a fairly good job of it. See
this dude's montage below:
Unreal Tournament 2004 might be a good choice too, as it's singleplayer instant action is second to none- though I don't know how many play it online these days (offline was always good enough for me in that one anyway)
There's also an indie game currently doing the rounds called Sub Rosa- which is a bit like Mobile Forces but only in cars, and with a business theme. Lots of
interesting ways that car chases and shady carpark trades can go down, especially if the third team gatecrashes the party guns blazing 
Just be warned the community can be fairly toxic at times.
