I confess I do have a soft spot for certain sci-fi, and I would guess I have seen every episode of every Star Trek series and every movie. Many of them, multiple times. They are always showing reruns of the various series around the time I am making and eating dinner and I always check in on them and refresh my memory.
Some nerdy comments:
The Next Generation - It had highs and lows. My favorite episodes were the ones involving Klingon politics and the arc with Worf's discommendation (sp?). Patrick Stewart is the best actor any of the series had IMO.
Deep Space Nine - It had some very good patient plot arcs. It also had some serious mis-steps (alternate universe, most things Ferengi) but overall was a very good under-rated series. The war with the Dominion was excellent. It was good in its own right and then they got the Klingon politics which I love in there too. That war was the highlight of all the series for me.
Voyager - Great premise, the best of any of the series. The long-term development of 7 of 9 was a highlight. I hated the hell out of Neelix and, to be honest, was not a big Janeway fan.
Enterprise - Like most people, it was my least favorite of the sequels but I don't think it was bad.
Original - What can you say? It looks dopey now - often unintentionally funny - but it was good for its time.
Personally I rate DS9 and Voyager above TNG.