Currently, I think it's TNG, followed rather closely by DS9.
A couple of years ago it was the other way around, maybe it'll swing back some day.
My explanation? While I'm capable of enjoying every incarnation of Trek to some extent, some shows just had a better ratio of good episodes vs. bad episodes than others. And VOY/ENT are just not as good in that respect as TNG/DS9. So, the reason is basically: better writing. TNG's writing was also a tad better than DS9's. On the other hand, DS9 had the more interesting (= better developed) characters. Hence it's such a tough call between the two.
Ah yes, and what about TOS? While I'm capable of enjoying TOS too, TV shows from the 60s are ultimately not really my kind of thing. It's not much more than that.
I'd also like to add that he final two seasons of ENT in particular remain a guilty pleasure of mine, despite their flaws and all.