Got a buy 2 get 1 free deal at a local game store, wondering what should i buy?

gonna spen 20 bucks or less so wat should i get may go higher it all depends, ill take any suggestions, heres the games i already played or got....Battlefield BC2, Alien VS. Predator, fable 3, rockband 1 and 3, call of duty modern warfare 2 and black ops, gta iv with expansions, borderlands, red dead redemtion, medal of honor, bioshock, halo wars halo 3 and odst, halo reach, looking for xbox 360 games,