I found the following information at MMOData.net. These are charts in which they track sub numbers over the years, measuring their peaks. It's a little out-dated, having been updated on May 8th. I just found it interesting, how many games didn't even breach the 1 million mark. Assuming this data is correct, of course.
1. Games that have peaked at over 1 million.
2. Games that have peaked from 150k-1 mil.
3. Games that have peaked between 150k-50k.
Edited due to large picture. Can click images for bigger versions. -Arlee