I think the Portal series is, overall, slightly better than the Half-Life series, if only by virtue of it actually having a definitive and satisfying ending. I also remember liking Angel more than Buffy as a kid, although I haven't seen either in a log time, so I'm not sure how they'd compare for me now. Then there's NCIS and Frasier, although those two have an easier time since I never had any interest in JAG or Cheers to begin with.
Oh, and Digimon: Tamers. As a child I enjoyed it for its shinier visuals and darker sci-fi tone, and as an adult I can enjoy it for its superior writing. I still love Adventure, and 02 had its moments, but they just can't compare to something that actually seemed to know where exactly where the story was going to go from beginning to end.