I think the whole Marty/Doc thing works because Marty doesn't seem to fit in all that well at school (Jennifer seems to be his only age-appropriate friend), in the same way Doc doesn't fit in all that well any
where; they're both lone wolf/solitary eagle types. I mean, I don't know how they initially
met, but their ability to maintain a friendship makes perfect sense to me.
I have only seen parts of BTTF and I think I'm the only one that isn't named Jezzem that thinks BTTF II was the strongest of the bunch.
I also think
Part II is the best one of the three. I appreciate Xanfor's point about it being inextricable from the rest of the trilogy, but I don't think it suffers for that fact. In addition to all the sci-fi goodness/coolness that M_R_M mentions, I appreciate its darker tone (the first movie is slightly wackier, and the third is a full-on romp*); it's the most emotionally-resonant of all three films, and as a super-sensitive girl that appeals to me.
*"Wacky" and "romp," of course, are not negative terms; I think the trilogy as a whole is pretty awesome, and all three movies are damn good individually.