Let me chime in with Babylon 5. The first half-season is so-so. But once it gears up, it stays up. (No sick pun intended.)
I've watched the entire series multiple times, and it still hangs together. From halfway through Season One to the end of Season Four is one of the best sci-fi stories ever told on TV. Season Five, while not quite up to par, is still great television.
Be warned, however, that B5 is more or less a single story. Sure, there are some standalone episodes, but by the time you reach Season Three, they're few and far between. It's basically a TV novel.
And G'kar? One of the best character development arcs in the history of cinema, I kid you not.