The 109G series is my favorite... the G2 and the G6 are very close in performance, which for both is awesome and well rounded. For me the G6 is even a bit more stable, I prefer the way it flies to the G2. However, guns aside the G2 does have a small but definite edge on the G6. Guns aside, that is... I like the added firepower of the upgraded cowl guns.
The G14 is a very flexible late war fighter, with the 20mm, 30mm, and 20mm trio packages, as well as it's ordinance options. Generally, I like to use the MG151 for fighters, 3 MG151's when I know I'm going to encounter almost only bombers under 20k, or BnZ, and the MK108 for mixed fighting and high altitude buff interception. The G-14 is a pig at high altitudes with gondolas (as is the G6, for that matter).
I also like the F a lot. It's just a well balanced, great performing, beautiful aircraft.
I don't like to fly the K as much as the others, it's a bit 'uber' IMO. Extremely fast, high rate of climb, etc. I also don't like the MK108 as much as the MG151. It's the easiest 109 to use effectively IMO.
If it weren't for it's lack of roll rate and MG/FF's I'd like the E a lot more than I do, but it's still fun to take up occasionally and TnB.