For me, the worst ending is easily Hibiki's. By that point the show had gone so badly off the rails that it barely made sense anymore and the last episode was a total mess.
For best ending I usually go to either Blade or Black. Blade's ending, of course, is famously tragic but I like how powerful it is that Kenzaki was willing to sacrifice his humanity for his friend. It was both touching and awesome. Black's ending is another great tragedy and I find it really moving that the last five minutes of the show is just Kotaro sitting alone in the abandoned coffee shop remembering all the friends he's lost. I don't know, I guess I just kind of love more emotionally complex endings (and yet W's ending is also way up their on the list for me because it was so happy; go fig).