I'll just leave this here.
http://sekijitsu.com/2012/01/03/weekly-art-fatezero/
As always, it's somewhat difficult to keep my experience with reading the light novel's second half from interfering with my perception the anime's first half, which is why I generally don't comment on anything anime related. That also, of course, means that I'll subconsciously tend to ascribe a higher level of favoritism to a particular character based upon what they do in the second half, as well as what's actually been shown in the anime. That said, attempting to put this aside and see the first half as separate from the second, my favorite characters are still Saber, Rider, Kariya, and Irisviel, in that order.
Saber hasn't really done anything impressive, so far, but I like her largely for the same reason that I like Kariya, in that she represents one of the few largely noble characters in a story with a a high dosage of borderline despicable characters such as Kiritsugu and Tokiomi. There's also the fact that I find her attitude towards kingship to be genuinely noble, while I regard Rider's as more of a character flaw. Sacrificing the lives of thousands of men, women, and children, through a series of
Wars of Aggression, is morally ambiguous to say the least, while Saber, as Rider was forced to concede, came off as a pure saint, who was so largely dedicated to her responsibilities as a ruler that she was willing to forego her own happiness. That level of sacrifice for a goal that is perceived as worthy, would be admirable even if it wasn't done within the best intentions to help her constituents.
Acting as a political figure is a high position of responsibility with an untold number of people, many of them qualified to do so, interested in filling that positions. In times, that responsibility means foregoing one's own happiness for a greater good, and I find it admirable that she was willing to do that. In such a position of responsibility, be it as a parent, a representative for a company, or a politician, living a "greedy", "fun", enviable life shouldn't even enter into the the equation.
A position such as kingship is indeed "thorny", but if someone is unwilling to forego some level of happiness in order to acquire whatever benefits they find in acting out such a position of responsibility, then they should simply step down or remain uninvolved, as many, more qualified people, would be willing to fulfill such a position as that of a ruler or a political leader. Still, though I don't agree with Rider's attitude towards kingship, the man is so thoroughly gregarious and accepting of others, that it's extremely difficult not to like him, based upon his personality traits.
I would vote for Kiritsugu and Kotomine, but the first half hasn't really covered them enough to justify it. Otherwise, I would select Kiritsugu as my favorite character with Kotomine leading a bit behind. Both are just extremely interesting characters, from a psychoanalytical viewpoint.