Next Gen is a slow build up as people have said, but it has some of the most memorable Trek episodes. Its biggest flaw was that it was not a continuing story-line for the majority of its run, and that it turned into a soap-opera in space.
DS9 was a slow build, but the Dominion Arc was beyond great (seasons 3-7). Seasons 1-2 are throw away, for the most part. The characters were the best developed and absolutely amazing, however. I didn't like all of them, but by enlarge, the main cast was the best. Sisco is Kirk, Picard, and Janeway combined, and better than either at their own game.
Kirk could fight, but Sisco could headbutt a Klingon, Jemha'dar, and Cardassians, and walk away unphased. Picard was a diplomat, but Sisco ran DS9, dealt with the Bajoran's on a regular basis, played ball with the cunning Cardassians, treated with the Founders, the Federation, and countless others; not to mention ended the Klingon War single handedly! Janeway was cunning and manipulative (in a subtle fashion), but Sisco was that to the pinnacle of nearing perfection. He duped the Romulans into joining the war against the Dominion by having one of their ambassadors killed and pointing the finger at the Dominion.
Voyager was terrible, with the exception of select episodes which were amazing. 'Two Years of Hell', I believe was the name of one episode (or two parter). It was excellent. The series finale was superb as well. Otherwise, the show had so many flaws and holes, a galaxy could fit through it. I really never liked any of the characters. They were just there.
Enterprise unfortunately suffered from the, "trying to make it look old, yet still looking better than original Trek" syndrome most prequels of older movies or shows suffer from. It also had poor stories. The acting remained basically the same. So it wasn't the actors fault. The writers were to blame. The last season, however, got good, but by then it was too late. It also started throwing in partially nude T'pol and Trip, which was worthless. Star Trek does not need nudity to sell itself. Voyager, actually did this too (in the form of skin-tight seven of nine outfits). DS9 dealt with sexuality, but really didn't have any nudity (just a few rare lightly clad ladies).
As for original Trek, I haven't watched many episodes. I can't always stomach late 60's early 70's corniness, which original Trek has in abundance.