The Space Pirates is what no TV should ever be - dull.
It's hard to judge it fully seeing as how 5/6 is missing, but the soundtrack is dull as ditchwater, the plot crawls along and few of the supporting characters are engaging.
The Underwater Menace is daft as a brush, the plot is beyond silly, the acting in places is atrocious, but it's still fun, and it does keep your attention, even if you are just wondering just how much sillier it can get.
The Dominators is also a rather dull story, but there's something that keeps me watching. I quite like the Quarks, rubbish robots though they are, I rather like the bickering of the Dominators themselves, surely the dopiest and most useless alien invaders ever, and I even like the premise of the story, the idea of a peaceful society being invaded. It's badly realised though, the cast don't seem to be terribly bothered, the effects aren't very special, and the whole thing seems rather amateur, but it's no worse than, say The Krotons, which is similarly slapdash in places. And by trimming an episode it isn't, crucially, as dull as The Space Pirates.