[Cal.]

Technically, through the stats, Cheryl is the best mentor (she's only judged a full 3, and ended up producing a winning act twice) and then her fourth series is still ongoing, but I agree, Dannii (in my personal opinion) is the best mentor. If Cheryl doesn't win this year (doesn't look likely), then her and Dannii will be matched as best mentor. Louis is the worst, with only 1 win out of a 11 series run. Simon isn't too good either, 8 series run and 2 wins. Mel looks set to continue the trend of a new judge winning.