http://tvtropes.org/pmwiki/pmwiki.php/Main/EnsembleDarkhorse

Essentually, an ensemble darkhorse is a minor character that becomes unexpectedly popular. Now, the BTTF fandom is relatively small - compared to other fandoms - so this would be a little hard to determine. The Family Ties fandom is even smaller, though.

Anyway, borderline cases might be Marty Jr and Griff - even though both did play pretty important roles. Marlene would probably be the better candidate, as we barely know anything about her - other than that she tried to break her brother out of jail, in the original timeline. Spike could be a potential one - as she is pretty hot, for a gang member - yet, Griff's gang members probably had the least distinctive personalities.

Of course, Lester the Wallet Guy might be the primary candidate - as he only had about three minutes of screen time tops, but he has a surprisingly memorable role.

Anyway, what are your thoughts?

From Part I:
Jennifer (She really doesn't have much screen time in the first movie but many people including myself still prefer Claudia Wells as Jennifer)
Goldie Wilson
Marvin Berry
the Bum
Would Strickland count here?

Part II:
Wallet Guy/CPR Kid

Part III:
Marshall Strickland

I suppose Strickland could, but he was a pretty important character. Although, yeah, I guess he really only appeared on thee occasions in the first movie - and on two occasions in the second.

Unnamed characters that could be possibilities:
Clock Tower Woman (well, she did hand Marty a flyer)
The Video Game Boys (especially Elijah Wood)
The Hoverboard Girls (the hoverboard had a bigger role, than the '50s skateboard of Part One)
The Undertaker (he's sort of the Wallet Guy of Part III)
The Colt Salesman
Barbed Wire Salesman (he's the Clock Tower Woman of Part III)