I really like G3, but, of course, I haven't seen the others. I bought G3 as a result of using it on a friend's computer. It seems to have a very good physics model behind it, and it seems to fly realistically to me (though I have little flying experience). I also consider it a work in progress, since it is constantly being improved, upgraded and debugged. (In fact, there are two new versions available as beta versions (v.426 and V.430) over and above the official v.410. According to those who have gone to the beta versions, they are significant changes. In addition a new "pack" is coming out at the end of this month or the beginning of March according to Great Planes and the software developer.
The only downside that I can see to G3 is the fact that it takes one helluva graphics card to run it up to its full potential. I get by pretty well with my card, but I can't get shadows, for instance. All else seems to work beautifully.
From playing with another friend's G2 version, it appears that G2 lacks a very important feature that G3 has (unless I was playing with an older version of G2). G3 has a feature which allows the ground to always be in view. This gives the flyer a reference point at all times. G2 doesn't seem to have that feature. If I'm wrong, I would appreciate a correction. My buddy said he couldn't get a "ground always" view. However, in terms of physics, I really didn't seem much difference between G2 and G3.
Harlan