The two algorithms I have been getting a ton of mileage out of lately are Bayesian Bandits and variations of the TrueSkill ranking algorithm.

Bayesian Bandits Explained:

https://www.chrisstucchio.com/blog/2013/bayesian_bandit.html

TrueSkill Explained:

http://www.moserware.com/2010/03/computing-your-skill.html