I've only seen New Voyages and Hidden Frontier.
Hidden Frontier has the look and feel of a fan series. The storylines and dialogue are very similiar to any fanfic story out there. It is what it is, a bunch of fans making their dream of creating Star Trek a reality.
New Voyages feels like a continuation of the original series. The appearance of former crew members (professional actors) helps. Most of the stories have been written by bona fide writers also. It is camp but it's supposed to be camp.
Gotta vote for NV.
They had no guide for putting me back together