The 20 for me as a favorite just because it is badazz with no other models coming close to the shear power it has. Especially equipped with a 6" LWD fully supported barrel and topped with a fast acquiring red dot sight.
10mm is awesome.
I have a G19 and a G27. its pretty hard to decide... I would have to say the G27 due to it's concealability and the fact that it can shoot three different calibers simply by changing the barrel and magazine...i have thought about replacing my G19 with a G23 because of the conversion options.
I have a G20C and a G17, the 10MM has a KKM barrel coming soon as they get them into stock... I would probably pick the G20C as my favorite Glock but it is not my favorite handgun of my collection. My Dan Wesson Pointman Major in 45ACP is my favorite and most accurate....
I'll always have love in my heart for the 23, because it was my first gun. I will also always love the 27, because it was such a joy to shoot.
As far as current favorite, though, I'm torn between my 29SF and the pure power of the 10mm and the 37, which is just a fantastic and accurate gun. I think if I really had to pick one I'd have to pick the 37, only because I'm finding the 10mm to be a little hard on my hands these days.