Mass Effect 3 is $8 right now on AMZ. Fun MP and all MP updates are free (thanks Bioware).
I would rather play gears of war 3 MP than judgement and Gears 3 can be found for less than $10. Its still very active as many didn't like judgement MP and went back to gears 3.
But the others have said most of what I would recommend.
Between BF3 and BO2, it is largely dependent on your play style.
BO2 is more just run and gun. No vehicles. Based more on reaction time than setting up perimeters etc. its largely considered the dumbed down game. Which I have no problems with. Also the maps so far aren't really set up well for sniping as many are close quarter combat with only a couple sight lines which get attacked often. So this leads to "quickscoping" in which you can learn to pull the trigger while you a scoping in (not sure if you played previous CoDs). But 3arc originally took out quickscoping when BO2 launched, but they complained so much that they brought it back. Its harder than previous iterations meaning less people doing it, but its still annoying as heck when it happens to you.
BF3 can have more of a complex, strategy based as buildings are collapsible, vehicles etc. likewise some of these maps are made close to the same as their PC versions which allow up to 64 players. So you can imagine that with an 18 (think its 18 or 16) player cap that these maps are quite open. Usually meaning a whole squad or 2 are just snipers hiding in the back sniping the entire middle or just getting vehicles.
Oh and don't forget, BO2 has zombies. A great survival shoot em up zombie maps. Plenty buy BO2 just for zombies.
To each their own. I enjoy BO2 more than BF3, but they seem to take 2 different routes of FPS