As the topic says, what GTX 980 under 600 is the best (most overclockable). I was going to get the MSI with a badass custom backplate from V1Tech but I like what I have read about the G1. I am not waiting 3 plus months for the 300 series from AMD, if the 300 series is that good I will sell my 980 and get a 300 series card. I had 970's but returned them because of the 3.5 VRAM issue (even though my G1's were freaking awesome).
I've already done the crossfire 290x thing and I don't wan to do it again.
I like the EVGA 980 acx 2.0 Superclocked. I hear good things about the acx cooler, and I feel that factory overclocked cards are easier to further overclock.
I have one of those, mine runs at 1392MHz out of the box. The cooler was very quiet, however it ended up back in the box because I decided to put a water block on it.
What games you running, and how is it performing?
Recently - Metro Redux, Crysis Series, Farcry 3 and Shadow of Mordor. Running at 1920x1080 constant 60 FPS maxed except no motion blur (I just don't like motion blur) or SSAA.
Simple. Go for the Gigabyte GTX 980 G1 Gaming.
I would seriously consider an R9 295X2 if your are looking for performance. It should crush any GTX 980 in pretty much any AAA title.
But if you have to get a GTX 980 I would probably go Gigabyte G1 Gaming right now.