I am also one who thinks of them all as a continuous story. So I voted for all three equally. Taken individually, I love just about all of the first one, except I do feel the "Johnny Be Good" sequence drags a bit and I can never buy into the starter failing just as the alarm clock goes off and yet Marty still manages to hit the wire at the correct second. The very end also kind of bugs me, that the improved 1985 was such a vision of '80's materialism. I used to like the second one less, but in recent years, it's gained stature for me. I actually think the ending of II is one of the greatest endings of any movie ever. The instant shock of the DeLoreon being struck by lightning and it looks like Marty is stranded, but then the Western Union guy shows up (I'm sure they'll hold a letter for 70 years and then deliver to a random guy on a length of road somewhere that probably doesn't even exist yet.... sure) and then Marty runs off--"I'm back, I'm back from the future"... great stuff. And I actually don't think I have any problems with the third.
I also love the musical score. Alan Silvestri nailed it with the themes in this picture. Especially the first one. If you look at what's happening, there are a handful of big effects moments, but most of what you're seeing is actually a bit low key, yet the score punches everything way up and gives the whole thing a way more epic feel. I would argue that it might be one of the top five most effective scores in any movie.
All around a great series of films. If you think about what's going on, the logic of the time travel is nonsense. This is why I love the locomotive time vehicle at the end so much... it doesn't make a lick of sense, but it's awesome anyways, and that makes it a metaphor for the entire trilogy.
--Alex