I don't know what the best one is, but I have long hair (I just got it cut, but it was about half way down my back), and my flat iron is perfect. I have really, REALLY thick hair, and it still works very well. Its a CONAIR. It also has a curling iron that you can lock, so its there when you need it but it won't get in the way. It also has a lot of different heat settings, so you don't have to worry about burning your hair off lol. and it also has a good long handle. I always burn myself when I'm using my friend's irons or whatever, but I've never burned myself on this one.
Here are some pics I took for you. The first one is just the straightner, then the second one is the curling iron.