James died trying to buy Lily time to escape with Harry. Arthur is also a good father, and while I'm sure Remus would have been a good father we didn't see much of him with his son. Vernon and Lucius, while they did love their sons and did their best to bring them up well, allowed their sons to grow up as prejudiced bullies.

Easily, no question. But also James & Remus.

It's hard to compare him with James and Remus though, seeing as we never really get to find out what they would be like as fathers.

James sacrificed himself for his son. If he valued his life more than Lily's and Harry's, his instinct would have been to attempt to make a run for it, rather than scream at them to make a run for it while he stood wandless to hold Voldy off. But I'm sure all the dads on this list would have done so too.
Arthur is a good dad, but what put me off about him as a father is that when his underage sons flew his flying car, his response was "Did you really? Did it go alright?" I mean, I understand being a lenient, laidback parent, but to be be that lenient when his sons did something that could have killed them was a bit odd even if it was funny (In my opinion).
Remus seems like he would also have been a good dad. Lucius and Vernon were good dads, but they didn't raise their sons right.
James was a doting dad who conjured colorful puffed smokes to entertain his son.