Natural Selection 2, Portal 2, or Arma 2 CO (for Day Z)?

I've never played Natural Selection, so I can't help you there. Between Arma 2 and Portal 2? I say Portal 2. Fantastic game. I had trouble getting into Arma. The actual game or the DayZ mod.

I'd go with Portal 2. Tat game alone gave me 50 hours of pure joy and fun (4 playthroughs, 1 in co-op and tons of cusotm maps)
Dayz will become standalone and eventualy more stable than current release. NS2 is good, but if you don't have a regular team to play with, it can get annoying really soon.

I would say Portal 2. Natural Selection and Arma 2 CO are crap, thats just from my opinion.

I'd say wait for DayZ standalone if that's the sole reason you'd buy Arma II CO. As for Portal 2 vs Natural Selection 2, it really depends on personal preference. If you like puzzles and humor, you'll like Portal 2. NS2 is great as well, but it's very different to regular multiplayer FPS games. It has an RTS element to it and you rely on having a decent person fulfilling that role. I have both and enjoy both,

portal

Natural Selection is a great game, but it's multiplayer only and has a somewhat steep learning curve. Portal 2 goes on sale all the time, and this is the first time NS2 has been 75% off.

Portal 2.

If you like multiplayer, NS2. If you like puzzles, Portal 2. Arma 2 CO is very good and fun, but if you only want to play Day Z you may as well wait for the standalone.