This changes, but it shows under "resoultion on the list what the best ones are
http://store.steampowered.com/hwsurvey/
1024 x 7681.43%-0.01%
1280 x 720 0.63% -0.04%
1280 x 768 0.30% 0.00%
1280 x 800 1.41% -0.05%
1280 x 1024 3.60% -0.12%
1360 x 768 2.69% -0.04%
1366 x 768 22.97% -0.29%
1440 x 900 4.38% -0.01%
1536 x 864 0.86% -0.06%
1600 x 900 5.69% -0.06%
1680 x 1050 3.35% -0.15%
1920 x 1080 46.17% +0.91%
1920 x 1200 1.13% -0.05%
2560 x 1080 0.55% -0.01%
2560 x 1440 2.00% +0.02%
3440 x 1440 0.25% 0.00%
3840 x 2160 0.82% +0.04%
Other 1.77% -0.06%
Thats from April of this year.