I loved OAV 3 Zhetto in concept, but not so much in execution. I thought there was so much potential in the concept of a guy who parallels Tenchi in many ways, except that his powers and his experience with the Chousin has only brought him misery and grief (as opposed to the happy life such things have brought Tenchi). And in his grief, he seeks revenge on Tokimi and her sisters by trying to screw up their plans.
But what really had a lot of potential was the fact that Z didn't actually hate Tenchi. Instead, he saw Tenchi as a fellow "victim" of the Chousin's meddling, and he views killing Tenchi as a way to free Tenchi of this percieved misery. I found that so much less cliche' than what your average fanfic writer might have come up.
It's a real shame that these concepts weren't developed to their full potential. I guess Misao and Masishu's romance was just too important to allow Z's story any time
edit: Oh yeah, and I as well prefer Z's design in OAV 2 over his OAV 3 design.