Out of all of the Servant classes, including the extra classes like Foreigner, Ruler, Avenger, etc., my personal favorite has to be the Berserker class. I just really like them for the most part, probably because I can kind of understand the idea of being so infuriated or mad that you practically power up and attack things around you.
My favourite class would be the Archer class. In most things I prefer ranged weapons above all else, and I like most of the Archer servants too. The Lancer class is in a pretty close second though. It's not often you see Irish mythology around.