Part 2 from 1986 is my favorite, more than the original even. Not a better movie, I do like it more though. It was one of the first movies I had when I was younger (probably too young to be watching stuff like this, ROTLD and Hamburger The Motion Picture - my early movie collection). I used to have a 6 ft Leatherface cardboard cut out my mom got from the video store when Part 2 came out on video.
I think it has the best looking Leatherface, every word out of Choptop's mouth is hysterical, Dennis Hopper is ridiculous, and that underground lair looks amazing. It's an amazingly entertaining movie in my opinion.
While I'm here, some brief thoughts on the others:
3 - I don't remember much about this one, need to check it out again soon I think.
4 - This one sucks, but I do love movies that suck, so it's at least pretty entertaining
2003 - I like this one, though I haven't seen it since probably 2003. I think Jessica Biel's overall ridiculous hotness might've helped my enjoyment.
2006 - I didn't hate it,but I don't like it much, it's such a bummer. It's brutal, but I think a little too grim and mean spirited. Of course I haven't seen this since it came out either, so I might need to check it out again some time.
2013 - This one is dumb as shit, and they kinda wasted the carnival scene, but I really enjoyed it. I'm a big fan of bad slashers like Killer Workout or Fatal Pulse, so stuff like bad acting, stupid dialog or absurd plots don't bother me a bit.