I voted for 2 but have love for 4
I'm a long-time fan of 2, in fact I think it's the perfect date movie. It has comedy, romance, bludgeonings, chainsaw dry humping, lots of Tom Savini gore FX... What more do you need for a snuggly night in?
But I really like how almost each movie in the series (that I've seen - 3D hasn't made it to these shores yet) has its own identity. The Beginning is a little too close to the remake, but otherwise all of the movies are so very different.
I think I like Leatherface the least because it's too obviously a calculated attempt at a slasher franchise builder, and the post-production woes left it with too damn many stupid continuity errors. It's always nice to see Ken Foree, William Butler and Kate Hodge though, and I think that Jeff Burr is a pretty good director (I'm probably one of the very few people to watch Pumpkinhead 2 multiple times). And yeah, I've seen the workprint.
I like The Next Generation quite a bit, especially when the guy in the limo shows up. Seems like Joss Whedon & Drew Goddard might have borrowed an idea or two from this for the overrated Cabin in the Woods.