Technically, I think Enterprise would be the worst the series has to offer.
The original series is, of course, a somewhat campy classic and basically mandatory sci-fi viewing.
TNG is also a classic, although its success is largely due to Patrick Stewart bringing the Shakespeare; it's about half-and-half really neat "
idea"
episodes and nearly unwatchable preachy or unbelievable alien culture episodes. Still, the strongest episodes of any of the Star Treks were here.
DS9 is my favorite of the five; it's the only one not to be entirely shiny and optimistic, and keeps a partially successful story arc going. It's particularly interesting for being the former home of the creative staff behind Galactica, which by all accounts wasn't entirely satisfying either for them or for the network suits trying to keep it accessible and largely dumbed down. Still, many of the themes of Galactica can be seen here in early form - difficult politics, mystical overtones, ambiguous characters, and "
let's see how much crap we can pile on O'Brien before he cracks"
. It's at its best when explicitly undermining the Next Gen utopian society, and at its worst when doing the Ferengi episodes.
Voyager is perfectly watchable, but is pretty much by-the-numbers Trek with a vast excess of technobabble (which is a horrible misuse of its really neat setting). It's precisely designed to be the sort of thing that can be tuned in by casual viewers and watched instantly - shallow but flashy plots, one-dimensional characters, and an aggressive avoidance of any sort of continuity between episodes. Returning to Galactica, it's probably more interesting when viewed as what that show was specifically reacting against - in principle, one could have gotten something very similar to Galactica from the premise set up in the pilot, but they opted for generic Trek instead. Oh, and they neuter the Borg.
Enterprise should have been good, but ultimately never justified its own existence. The pilot is worth watching, but that's about it.
For what it's worth, I own the Next Gen and DS9 DVDs, and watch the original series and Voyager when I run across them on cable.
____________________________
Click here to view the secret text