Fox has a mixed record, they put a whole lot of Fantasy/Sci-Fi programs on the air to begin with, way more than any other network, but they often mess with them big time, or cancel them quickly. It's like a Love/Hate relationship.
NBC wasn't exactly kind with how they abused SeaQuest and took it from being a pretty hard-science show into later being a "monster of the week" program. They also were responsible for slashing (Original) Battlestar Galactica's budget so harshly we got the atrocious Galactica 1980. They aren't innocent here.
UPN turned Voyager into the "Ogle Jeri Ryan" show, then abused Enterprise and ran it straight into the ground with atrocious scheduling. They also gave us "Homeboys in Outer Space", which is a separate offense.
TNT actively sabotaging and scuttling Crusade was their big sin, but they haven't had the repeated offenses of other networks. They aren't much of a Sci-Fi network in general though, they are more of The Law & Order Channel, that dabbled with B5 for a short while.
Of the four, I'll go with UPN, since they trashed the entire Star Trek franchise, Fox isn't blameless but they do also a lot of good to balance the shows they've wrecked, UPN hasn't gotten a single one right, ever.