I played 2 the most. The only irritating aspects were the lower range pistol and glitch-jumpers on ascension. Overall the weapon balance was pretty well done. Maps were the best by far. Swords were annoying at close range but not unbeatable by any means and offered great potential for killstreaks. Dual-wielding offered more options in terms of taking down shield versus health. I can see why they took it out though, although the addition of the DMR kind of renders the pistol obsolete again.
I don't know if that means 2 is the best, but it's the one I'm most familiar with.
The dash feature is my main gripe with 4.
And what is good, Phaedrus, and what is not good - need we anyone to tell us these things?