"Voyager" is the easy choice for me. I've only seen like six episodes, and there wasn't a single one that I thought was better than 'has a few good parts'. None were consistently entertaining all the way through or particularly excellent or memorable. Any episode I've had to watch of that show was just hard to watch, and I thought most of them were really boring. I didn't like any of the characters either. I've seen even less of "Enterprise", but I was so blown away by "In A Mirror, Darkly", I have to rank it higher than "Voyager" as a whole series just on the basis of those two episodes.