Well I have Gen 2,3 and 4 and like them all. I'm getting to like Gen 4 the best. I'd expect most people would still like Gen 3 the best.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek
The gen4 has some updated features for every shooter plus you can change the mag release to work for a left handed shooter. I think the RSA has also been upgraded and there has been a voluntary recall/ trade out of the RSA in the early Gen4 models.