Louis is the worst judge - he never has anything constructive to say and there is seldom anything in his 'critique' which is helpful or astute.
Tulisa very often seems to have her mind made up before she starts and has obvious favourites, which she freely admits, but it does make it difficult if you are not one! I think this comes from her having less experience of different musical styles.
Kelly has the ability to be a really good judge, but only occasionally gives the contestant the benefit of a full critique - all too often she comes out with meaningless comments.She has a better knowledge of American music, so sometimes can be unfamiliar with some of the songs performed.
Gary is the best judge - his comments, although often harsh, can be spot on and he doesn't favour anyone in particular, he remains reasonably impartial (apart from his own acts obv.) He has a good over all knowledge, and is not stuck in one particular genre.