All those GOW supporters are just picking it because it's free.
For me, it's definitely R6:Vegas 2. I played R6:Vegas for the first time last month and thoroughly enjoyed it. R6V's perfect blend of realism & action, huge arsenal, and fluid control mechanics easily make it the best cover and shoot game on the market. In fact, I probably wouldn't have enjoyed Gears or Uncharted at all if I had played R6V first.
My 2nd choice is Condemned 2. I loved the atmosphere of the 1st game although the gameplay did get a little stale at the end.