The first film is relatively straightforward by time-travel-story standards. Marty McFly simply has to 'fix' the the timeline he has inadvertantly screwed with, and manage to get back to future, or present (though Back to the Present doesn't sound as good). Overall, I feel that this film is the strongest of the trilogy as everything holds together well and I have no particular criticisms. The scene with the clock tower is thrilling, and the final few seconds of the film are fantastic (and I'm not sure how I avoided having them spoilt for me).
The second film is probably the most rewatchable as it has the most labyrinthine plot and some really smashing visuals, particularly in the 'future' segment. However I also feel that it is ultimately the weakest of the three as greater emphasis is given to the series' antagonist, Biff Tannen. Through no fault of Thomas F. Wilson, Biff is a pretty weak villain. He's completely one dimensional and, worst of all, not all that threatening. Perhaps due to this, there weren't any scenes that felt particularly suspenseful to me. In the first film, Biff's the stereotypical Hollywood jock, but by the second, his evil jock persona has ballooned out of all proportion. For me, Biff is not a strong enough villain to build an entire film around, though a valiant effort was made.
The third film has some great elements, but is held back by some noticeable flaws. Introducing a more distant historical setting is a nice change of pace, and the old west is realized in a way that is both tongue-and-cheek and believable. I love that Marty's historical equivalent is also played by Michael J. Fox. The Biff equivalent is still weak, but downplayed from the second. The big problem is that while he was never mentally intimidating, here he no longer seems physically intimidating either. And while the fairly straightforward plot holds together pretty well, there's not much of a sense of urgency about anything since there's no particular hurry to get back to 1985, and no apparent threat of messing with the timeline. In essense, Biff's role as a source of tension has been dialed back with nothing dialed in to replace it, though in my opinion this still makes for a stronger film, and a good amount of excitement is generated in the climax. The movie does have one extremely bothersome plothole though. Early on, they save Clara from plummeting off a cliff, but are mildly concerned that this may have disrupted the timeline after discovering that she was indeed supposed to die. After she and Doc Brown fall in love, Marty suggests bringing her back to 1985, but the Doc dismisses this idea as he is afraid it will disrupt the timeline. No!!! Leaving her in 1885 is what will disrupt the timeline! Hello? Hello?! What's worse is that this is never pointed out, and the entire 'disrupting the timeline' idea is dropped like horse pucky. Come on, you're smarter than that, Doc. Think, Brown, think!
Overall, the series is pretty solid. All three films range from decent to great, Michael J. Fox and Christopher Lloyd are both endearing and enduring in their roles, and the only big problem is that the sole antagonist is not intimidating, although this isn't really a problem in the first film. I think it would have been interesting if, for the second or third film, they'd introduced a rival scientist, or maybe a cruel version of Marty from an alternate timeline. Ultimately though, Back to the Future is the rare film that is both enjoyable for adults and appropriate for children, and it is no small feat that they were able to duplicate this achievement two more times.
I'm interested to try the new adventure game that has come out. It's nice that Christopher Lloyd is reprising his role, and the guy doing a Michael J. Fox impression is outstanding.
I close with the immortal words of Sam Neill: "Where we're going, we won't need eyes!"