The Empire Strikes Back for me.
It has the best score in the series (seriously, you can't go wrong with introduction of such themes such as "Han Solo and the Princess", "Yoda's Theme" and the "Imperial March" among others) which manages to elevate the film into a totally new level.
The triangle drama is commendable, and the growing relationship between Han and Leia is quite bittersweet and gets a dark turn of events late in the film with the heartbreaking carbonite chamber scene. The battle and chase scenes are memorable, such as the Battle of Hoth, the asteroid chase and the duel between Luke and Vader. The duel in particular contains one of the most shocking twists ever in film history, and overall its both intense and emotionally gripping as both a physical and emotional struggle.
The special effects look neat and still don't look that dated and instead help to carry the plot along, and a big thanks goes to director Irwin Kershner who wanted to emphasize this aspect. The dialogue is quite sharp and full of memorable lines, and the actors do a great job in their roles. The characters themselves are great, and I like how Vader gets more screentime and is an all-around badass in the film. The introduction of new characters such as Yoda, Boba Fett and Lando is great, and they add depth to the plot. As for the plot, it works quite well as it portrays a very dark chapter in the saga's history, and it's among the darkest and most mature of the Star Wars films.
I guess I could go on and on about the film's strengths, but I'll stop here. In any case I consider The Empire Strikes Back to be the best Star Wars film to date because of the many reasons I mentioned earlier. It works in so many levels that it becomes a good film on its own right.