Alrighty, great topic!
Episode II will always be my favorite. So much crazy crap happened in it, and despite a forced and unbelievable love story there were so many other great moments, new characters, and epic storytelling that bridged the gap between the old and new trilogy. Plus, Geonosis will forever be my favorite Clone Wars battle.
Episode VI is my second favorite, also because it added so much lore, and really feels like an epic conclusion. The Tatooine plot is my favorite part of the movie. So cool!
Next comes Episode VII. Incredible. Maybe a bit obnoxious with the Starkiller Base and a bit more fanservice, but no doubt still a direct and respectful addition to the series, and adds a lot of fun new characters to the mix. But dang, Lucasfilm chopping the EU and getting rid of 50% of my childhood...that hurts.
After that, controversial choice but Episode I. This was my childhood. Lightsaber battles, armies and an entire exploration of how the cleaner side of the galaxy lives, this movie is actually quite great. There are so many new creatures and species added, and for the most part the CG isn't overdone.
Then Episode IV. Classic, such a good movie, my first Star Wars movie, actually. It just gets a bit long and boring at times. Lots of dialogue.
My least favorite classic film was Episode V, it just never felt complete, and didn't add much to the universe in terms of creatures and lore. It feels like one big bottle episode.
My least favorite movie in the franchise is Episode III. It is a story that should have been built up in film and shouldn't have relied on the EU publications to help fans appreciate it. As a longtime star wars fan, it is still really hard to watch so many awesome characters with backstories and actual relevance (Grievous is TEXTBOOK to this argument) get one or two quick scenes that don't establish their characters and then have them get killed shortly afterwards. It's unbearable.
Note should be taken that if you haven't, the original Star Wars Clone Wars series that was animated a'la Samurai Jack/symbionic titan style is well worth watching. The two seasons last as long a movie, and are FANTASTIC. Especially in Lieu of Episode III's rushed plot.