It's really hard to choose, since they're all so inconsistent. The Reeves series was best in its first season but got increasingly silly later, and it suffered from focusing mainly on ordinary criminal antagonists that didn't pose much of a challenge for Superman. Superboy had a weak first season but got significantly stronger, although I only managed to catch its later seasons intermittently. Lois & Clark was essentially three different shows from three different creative teams: the first season was a pretty solid romantic comedy with underplayed action; the second was a more balanced action/comedy but suffered from the loss of Lex Luthor and the Michael Landes version of Jimmy Olsen; and the third and fourth were an increasingly campy mess made by producers with no respect for the subject matter.
And Smallville... what a roller coaster. I'd say its first few seasons were generally quite strong despite the always-problematical "meteor freak of the week" approach (really, if kryptonite were that ubiquitous in Smallville, Clark would've already died of space leukemia from all the years of inhaling its dust in the air and drinking kryptonite-contaminated water). But then it started to go downhill and was downright terrible by season 6-7, to the point that I actually stopped watching for a while. It resurged somewhat under new showrunners in seasons 8-9, albeit as a totally different show than it had been before, and then largely fell apart again in season 10.
So I can't really pick one as my favorite, because there isn't a single one that I have a consistent opinion of throughout. They all kind of run the gamut from good to bad within themselves.
My sentiments exactly.