TNG- First Trek show I ever watched, and also the best. I love the characters, the stories, and all of the morals.
DS9- Lots of character development, terrific villains, and long story arcs that always made me want to come back for more.
VOY-I'm only two seasons into it, but I have to say it is improving. It may very well move to the number two spot as I watch more.
TOS- Historically significant, but it doesn't hold up too well today. A few real gems spread throughout a gazillion bad episodes. I much prefer the films.
Haven't yet watched TAS or ENT, so I can't judge them.
Whilst the thrid season of TOS was rather lacklustre, the first two seasons easily hold up well today.
But then again it is a product of the era in which it was made, and as such tells stories to a certain extend different from telling a similar story 20 years later, when TNG came out. Plus sometimes the order in which you watch can affect how you percieve each show.
the fact that TOS, was on it's way back to TV(before it went to the movies), 8 or so years after it was cancelled, says something about the show. Bad shows generally get forgotten about and fade into TV history. How many other shows from the 60's have been remembered as well.
Yes recently they have been a lot of remakes from the 60's/70's/80's as studios plunder their properties in order to make a quick buck.