Last time I played BF3 (sometime last year before I sold my PS3) there was pretty much no one populating the hardcore servers or the servers running expansion packs like Armoured Warfare. Just a word of warning there. I know it was PS3 and we're talking about XBLive here, but it's something to bear in mind- you won't find a great deal of players on most older online games.
That being said, I used to have Rainbow Six: Vegas 2 on my PS3 (came out in 2008, 3 years before Battlefield 3) and there were a lot of people still playing- although it was a really good game, one of the best in the tactical FPS genre IMO. You could customise your character's kit entirely, with a metric fuckton of camos, gear and apparel to choose from. A decent selection of weapons to choose from as well, just not a lot of customisation options- and the cover system! My god, it was absolutely brilliant! Here's a screenshot if anyone doesn't know what I'm on about:
You could blind fire, flip around corners and open up on the enemy, change stance while stacked onto a corner, reload, switch weapons, attach suppressors while in-game, shuffle along the cover and all sorts!
Here's a look at the customisation system too:
It was absolutely mega! You could choose shoulder, chest and leg armour, helmets, caps, bandannas, sunglasses, balaclavas, respirators etc for your head as well as a plethora of combat uniforms (no official names but some look very similar to real life clothing) and in so many patterns- it really puts a lot of modern shooters to shame, even now.
BF4 is close to it in customisation, but not quite close enough as every faction's character model in it has to wear sufficiently different clothing in their own countries patterns to differ from the other teams.