Which should I buy?

both

THAT MEANS ALL 3

none?

Bc2

bf3 was bundled and even free. bf: bc2 was €1/$1. bf4 is full of bugs as people say. tough choice.

BF: Vietnam

I would go with none of them.

BF3 was bundled, then free. Should be cheap to trade for.

BFBC2, from personal experience, isn't actually dead, and it's still pretty fun. Vietnam is relatively dead with only a few active servers at any time, although I got a good deal on it during a sale so I don't really mind.

Don't have any experience with BF4, although people seem to say it's horrifically buggy, poorly optimized, and all the maps get boring after the huge change happens. Also, 120 dollars including DLC and shit.

None. EA have ruined the Battlefield series.

Go for Bad rats.

Just go for BF3. The servers are still populated. But for how many years? Nobody knows...

You can get BF3 really cheap so go for it. There're a lot of players and thousands of servers. BFBC2 has better single player and BF4 is just crap. I don't really like any of these (played all of them though) but I suggest you to buy BF3.

BF3

Why do you want to buy one of those?

BF4 2.0 was released and fix Alot of bugs, so id go for that one :)

BF3, still full of players and after 300+ hours in it i can safely say that it is addictive.

BF1942

BF 2142

I enjoy(ed) playing all of them, I put the most hours in Bad Company 2.
Bf4 has the biggest number of active players atm so I would go for that one.
Paracel Storm is easily one of the the most beautiful and fun MP maps I ever played.

None. Play Battlefield Play 4 Free, its just as good as the rest of them but its free and thats the best part about it, very battlefieldy http://battlefield.play4free.com/en/

Bad Company 2, hands down. The last good Battlefield game EA released.