You can sort games by popularity, but it's unclear what that means. Is it number of downloads, number of views, or a combination of the two? Also, if I check my game page a few times a days, are those visits included?
How about score = truescore * 1 / x, where x is the number of days in a row spent in the top 100. This would have the effect that a popular title would continue to sink until it left the top 100, then if it was still popular, it would bounce back up, but if its popularity was due to its favorable position it would disappear.