TOS is viewed as a very progressive and hard (by TV standards) sci-fi show. It focuses little on character development or plot development in favour of exploring sci-fi concepts and the human condition. The wonder and strangeness of the universe and our place within it is the focus of TOS.
TNG is generally viewed as dry and intellectual sci-fi. It has fantastic writing and (some) great characters, but tends to be very stuffy and preachy with a few too many
Patrick Stewart Speeches. TNG is mostly concerned with examining political and sociological issues while being light on sci-fi concepts, character development, and story arcs.
DS9 is the soap opera of the group and my personal favourite. It has by far the best and most developed characters and a complex, serialized plot. On the downside, it completely abandons its science fiction premise.
Voyager is generally viewed as light entertainment. It has some decent characters, decent writing, great action, great comedy (including general lightheartedness), and amazing special effects. The problem is that it's dumb as a stump. It's a great show to watch if you turn your brain off first, but it lacks the intelligence that defines the Star Trek brand.
Enterprise ... well, Enterprise sucks.
As you can see all four Star Trek series of value have very different styles and appeal to different types of people, so you're not going to find many fans who agree on a best Star Trek. Although I believe DS9 is the most popular here.