Back To The Future, great start to the trilogy, movies in 1985, 1989, and 1990.

Rating: Null/10 (9 June 2001)

As of this date, the 3 "Back to the Future" films have these IMDb ratings -- #1 (1985) 7.8 of 26,091 votes. #2 (1989) 6.6 of 9492 votes. #3 (1990) 6.4 of 8230 votes. So, it is clear that the first episode is the most popular, no contest, in both rating and number of votes.
I, too, think #1 is the best. In fact, I think it should stand among the best films of all time. Not for any one facet, but for the total entertainment that it provides. Almost all of us dream of "time travel." BTTF certainly is not the first to explore time travel, but the way it does it is unique. Plus the fact that this is clearly a comedy, but one that is so well conceived and written that I never find myself think of it as a comedy. I've been a long time fan of Chris Lloyd's, since his days as Jim Ignatowski ("flower child" spelled backward), and he is so perfect as Doc Brown. And, of course, Michael J. Fox as Marty McFly "defines" a role as few have been defined in films.
The script is uniformly superb in all 3 versions, and to me that is what really makes them "believable" within the context of a pure fantasy. Plus, although all the "science" in the 3 films is "bad science", which for a scientist makes it even funnier, they don't overlook any details. They always offer a "science" explanation for what Doc is about to do, even though it is completely nonsense.
Tonight, at the end of #3, I first realized one of the reasons I like the BTTF films so much. Ever take a vacation to a totally different part of your country or world? And when you get back home after a few weeks, to the familiar, you sorta mentally pinch yourself and think, "Just two days ago I was riding a train through southern France, drinking wine by the river, and sleeping in the 200-yr-old silk factory turned home in the country." And you look around, you're back home to a totally different world.
That's what I believe Marty McFly felt when he arrived home, saw Jennifer asleep on the swing. He had just spent a week during 1885, evading a band of Indians, seeing his original ancestors from Scotland, fighting an infamous gunfighter, and making a harrowing escape from the past by being pushed to 88 mph by a train that blows up, and now he is back home to the familiar. We can all identify with that, and the end of each of the 3 BTTF films gives us that identification.
The original, #1, involved the existing friendship between Marty and Doc Brown. Marty accidentally gets sent back 30 years to 1955, and must enlist a young Doc's help to get home. Meanwhile he inadvertently gets his future mother interested in him, jeopardizing the future relationship of his "parents." The old newspaper accounts of the lightning strike of the clock tower is used as a source of energy to get Marty back home. His fight with Biff in 1955 has altered history and his 1985 family is apparently much better off as a result.
#2 hinges on Biff inadvertently getting a copy of an almanac for "future" events and he dishonestly uses future sports results to gain fortune and power, and drastically change the future. Marty and Doc have to retrieve the almanac before Biff uses it, to restore history to its proper order. It ends in a way that sets up the #3 sequel.
In #3 Marty gets a Western Union letter that Doc Brown had actually written in 1885 and requested delivery at the specific time in the future. Doc has ended up in 1885, is content, and insists that Marty not try to interfere. However, Marty discovers a tombstone and historical account of Doc being shot in the back over an $80 issue. So Marty has to go back and help Doc avoid that fate.
To me, the original is definitely the better of the 3. The second and third ones are totally different, and I would rate them about the same. The science is funky, a rigorous analysis can uncover a number of "discrepancies", but to me none of that matters. The BTTF films are just great fun, and the good guys always come out on top. And there's nothing wrong with that.
2015 UPDATE: I bought the 3-disk BluRay set and they really are an upgrade from the DVDs, better picture and sound.