So far it's been interesting how closely angriest and I match. When I was thinking about this topic, my first thought was 'something like The Footy Show.' Then I decided that I'd go for something I hated that I had actually watched more than a bit of.
There's V the Series from the 80's, which is a text book example of how not to make SF TV. Seriously, think of any mistake you've seen appear regularly in TV and SFTV, and V made them all.
Dollhouse I found to be misogynistic and painful to watch. It had it's good moments, but it really felt like Whedon's problem was that his whole starting premise was flawed and hard to build into a decent show.
But in the end it really is a toss-up between Star Trek: Voyager and Enterprise, because what I really, really hate in most shows is watching something that's crap because of lacklustre writing and missed opportunities - things where the show could easily have been good, because it wasn't a problem of budget and the like hurting it - ST:V & Ent have both in abundance. While they do have their occasional good episode (or in the case of Enterprise one good season, the fourth) overall both are weighed down by show-runners who were lazy, uncreative, and uninventive in the extreme.
In the end I feel I have to go for Star Trek: Voyager, since Enterprise did at least manage one good season.
ST:V also had the initial theoretical advantage over Enterprise in that ST:V had no history it needed to try and conform to. On top of that, you have a concept to die for - two crews with opposing viewpoints are flung to the far reaches of the galaxy and have to work together on board a single ship with limited resources as they make their way home through unknown space. With the effects technology of the day and such a fabulous premise just brimming with potential drama, Voyager literally had more scope to tell fabulous, stunning, and human stories than any Star Trek series ever created, including the original.
By the end of the pilot, both crews have agreed to work together under Federation guidelines, and they do so with barely a hiccup from that point on. The next two episodes are built around temporal anomalies, which many SF series resort to when they have no other ideas. Through the series, episode after episode hits the reset button so that by the last episode, the titular ship hasn't changed a jot, even with everything it's been through.
It ran seven years, continuing to do lame, half-baked, stories that usually failed to take advantage of a stunning premise. Being part of the Star Trek franchise, it had the chance and backing to do really brave confronting stories, and instead usually did run of the mill dreck.
And I can't help but wonder how many other well-written and thought out shows could have been made with the resources allotted to Voyager.
For a slightly more detailed rant, with extra swearing, look here.