The Original Series is good if you like pulp sci fi with an optimist flavor. When it's good, it's really good, but most episodes are meh at best for various reasons.
TNG takes TOS's formula and refines it in many ways. Less pulpy, slightly harder sci fi, generally better stories and writing. The first couple seasons and the later ones ought to be graded on different scales because early TNG is reusing unused material from TOS. Where TOS is good, TNG is great.
If you like the techno commietopia aspects of TNG but think its optimistic tone is too unrealistic, try DS9. It takes the premise of a federation founded on "naive" ideals and then flexes them in an existential war for survival. Imo, it starts out a little weak but gets really good once it finds its footing.
Voyager, eh, has some good ideas that never really get properly developed. Not terrible, but never really becomes too good either. Where DS9 toyed with federation values, Voyager intends to do the same by putting a federation ship tens of thousands of light-years from the federation, but the writers never really firmly decide what that means, so it gets unevenly applied from episode to episode, which can undercut the enjoyment for some because it makes Janeway come off like a hypocrite.
Enterprise isn't terrible but it stays pretty mediocre through its run. Imo it fails to capitalize on a lot of its theories too, but it's still definitely star trek. It succeeds in some areas and fails in others. Personally, I don't like Count Bakula as captain. A lot of the time he acts like an idiot and makes stupid decisions instead of principled ones.
I don't recommended any of the movies past Undiscovered Country, though Generations and First Contact are watchable, at least. The Motion Picture and Wrath of z Khan are probably the epitome of Trek, with Undiscovered Country following up close behind.
STD, Abramsdreck, and pre-emptively Picard don't exist.