The 109s are very rewarding if you take the time to learn each one- just because you are good in one type (or terrible in it) doesn't mean you are like that in all of them. You really should try them all for a while (like a week or two on just one type) before moving on- one or two fights aren't enough to base a judgement on.
109E has a good climbrate and great diving speed for a 1940 plane, plus a good top speed. Its only drawback is weak cannons with low ammo load.
109F is very light and has great acceleration, climb and top speed. It can be kind of hard to get the kill shot with it though, and don't get suckered into a turnfight.
109G2- faster than a 109F in all areas and just as maneuverable. Ability to carry gondolas and a DT, first one in the stable to have both. IMO, the best of the 109 series in comparison to its contemporaries.
109G6 isn't as fast, but is still pretty nimble between 250-400MPH, and has much bigger cowl guns, making it much easier to get a quick kill. Loses a lot of performance with gondolas on though. Turns surprisingly well for 1 or 2 revolutions with flaps out, as does the G2.
109G14 is the best of the G series- great speed, can carry a 30mm cannon, and climbs like a rocket. Stay in the vertical and go in with E and you can handle just about anything in the late-war set. Also is the best performing with godolas. Best buff buster, along with the G6.
109K is the one I cant figure out. Great speed, climbs like a rocket, and has a 30mm cannon standard, but it doesn't turn well. It excells in the vertical, but a low ammo load and poor turning ability limit your options in a dogfight. But its does some serious damage with a relatively light volume of fire. Fly it like a P-38 or P-51- stay fast and don't get suckered into a stallfight.