I never watched any of these.
Which one should I watch? Which one is the best?
Have you seen the original series?
Either way, just watch them in the order they were released. They aren't long movies.
Part 1 is a pretty straight forward adaptation of the first third of the tv show that ultimately suffers because of it. Not because there's anything seriously wrong with it, but because it missed the opportunity to be more of its own thing.
Part 2 is a club banger remix of the second third of the show that is 100% unique animation and features a "cover version" of the plot. This movie skips a lot of the melodrama the original series is famous for and instead makes an energetic spectacle/blockbuster type movie. And it totally works. And the animation is fantastic.
Part 3 is a bizarre twist that takes the plot (and setting) to a completely different place than the original. It came out of nowhere for a lot of people and the whole movie is vague and kind of confusing. Everyone's thinking (hoping) that we'll get some payoff in the next film. While it's a fine movie with some great animated sequences, the jury is out on this.
Nobody can make heads or tails of what Part 4 is going to do. I hope it's bonkers. I hope Anno gets us good.
Tonally these films are pretty different from the television series. They don't focus as much on introspection and the main thematic material that is the real backbone of the original series is only implied (or absent), rather than forced to center stage. A lot of people really don't like the changes that have been made.
In one of the earlier DVD notes Anno says that he wanted the Rebuild movies to be like riding a train to a place you've never been before. When you first leave the station, everything looks familiar, but the further along you get the more alien and unique your surroundings become.