Love both, but I have to say Star Trek. The Twilight Zone had so many wonderful stories, and I’d say the ratio of good to bad for both series is about the same (the original Star Trek had a wildly uneven last season, Twilight Zone had a lame one-hour final season), but Star Trek is much less “dated” and stands up to critical scrutiny better than The Twilight Zone.
Too many Twilight Zone episodes had a paper-thin premise, and many of them fall prey to the “surprise!” ending that is often obvious to anyone paying attention (“They’re Adam and Eve!” “They are really aliens heading to Earth!” “The tiny aliens are really Earth astronauts!”) and so on. Star Trek had some thin premises as well, but they tended to make the best of what they had to work with, and they also had the benefit of established characters that we grew to know over the series…