Voting on this depends how you look at it.
In terms of actually being entertaining as a judge, Simon Cowell in his peak years (not the sad parody he has become since his return) was easily the best - he made the show, simple. He was always 100% honest, even if his delivery was rude.
In terms of being a good mentor / constructive judge, Danni Minogue wins hands down. But I never found her particularly entertaining.
Whoever is voting for Louis Walsh wants shooting.