To me, it's Finn. It's ridiculously amazing how many moments the two of them have had throughout the three seasons. Will got him in the Glee club by tricking him with that pot and stuff. And ever since the baby drama, being comfortable in his [Finn's] body, finding out what to do about his future, and beyond...it's like Will is Finn's new dad or something. Just sayin'.
I think that it's Finn, Will has helped him out a bunch during the whole series and seems to worry about him more then the rest of the club. Not by much, though c: He looked after Finn the whole series and broke rules trying to help him, like the pot thing c:
Kurt comes in a close second though (:
Most defiantly Finn. Finn was looking for a father figure and the two of them bonded over their baby drama. He let him drive his car, in the first season. Their relationship never really faded any.
Will needs to find friends his own age as some of the relationships are a bit inappropriate with a teacher, but I can think of a few teachers like that when I was in high school but I was invisible to most of the school in high school.