I'm sure it's been named countless times already, but it sure seemed like Deadwood had at least one great season left in it. As for other HBO shows, i have some friends that loved Carnivale, and say it's a shame that it got cancelled so soon.
Back in the day, Sports Night seemed like it had a lot going for it. Too bad it could never find an audience.
Comedy Central for a time seemed like it had a knack for cancelling it's most promising shows. Ex. Jon Benjamin Has a Van, Dog Bites Man, Michael and Michael Have Issues, The Naked Trucker and T-Bones Show, Sports Show with Norm McDonald, Stella, etc.
Adult Swim does a commendable job of letting their new programming find an audience, but I sure wish they would've made another season of Stroker and Hoop. That show was great and getting better.