The EJC Fitbit Alta Band is the best one I have tried so far. Just like the original band it is made of a flexible silicone like material with an angled pattern. The big difference is that instead of the standard clip it uses a buckle like a traditional watch. This makes the Alta much easier to put on and near impossible for it to fall off without you intentionally removing it. The buckle has a nice polished finish and so far seems to be holding up extremely well. The clips that secure it to the Alta seem to be made well and the fit is very tight. This band allows for quite a bit of adjustment and should fit any size wrist.
If you are interested in the EJC Fitbit Alta band it can be found here.