I grew up on TOS -- I was always stunned (even as a kid) that
Lost in Space constantly beat it in the ratings!
ST: TMP (aka Star Trek 1) was okay, but no great shakes
ST2: TWoK was great -- it worked best if you knew the original story, but it also is a taut film all on its own.
ST3: TSfS -- ummm, it gets you from 2 to 4. Essentially it would have worked better as a (shorter!) tv episode, but as a film it lost me.
ST4: TVH -- this one was for all the old fans! Kirk & Co being themselves under strange and silly circumstances. For me this will always have a special place because I was living in New Hampshire and was homesick for the San Francisco Bay Area, so...

ST5 -- forget it. Didn't exist. Move along, nothing to see here...
ST6: TUC -- ...wow... This is a great film for me. Timing, of course, is everything. Watching this film in the waning days of the Cold War when it looked like the old Soviet Union was going to shatter into a million component pieces, this is essentially what Star Trek did best -- take a contemporary issue, put it through a sci fi transmogrifier, and then look at it with a bit of a distance. Well written, exciting, tense, funny, this is the best of the Trek films for me.
I have seen the first two TNG films. The first one (Generations?) was bland, like much of the show. The second one (First Contact) was great fun, but I was bored with the whole Borg aspect. I haven't seen the other two.
For me, the best series is DS9. Complex characters, long story arcs, moral greys, conflicting "rights" and "wrongs" amongst essentially good people -- this was great television. Probably also explains why it is the only tv show I own a full run of

(HATE the packaging, though!)