Here's some criteria to think of.
1. People who have had championships in three or more different games (need not be at the same time) should definately be considered.
2. People who have had more than one championship at the same time (2 or 3 seems most appropiate as a minimum) should definately be considered.
3. Players who have had major impacts on competition early in their career (claiming lots of records early on, dethroning long lasting records, or even better, dethroning some of the longest reigns as champion in the site's history at the time) should be considered.