Man this is one hell of a question. There are hundreds of changes between them. I don't have the time or energy to try & tackle this question. Its best to know "Perfection is in the making" and let it go at that.
If I were to pick a single generation I would go with the Gen3 guns. They were the culmination of all the best ideas and have had a lot of time to prove themselves. The Gen4 guns may be the best ever but its going to take a lot of time to prove it.