Can say I liked all 5 series (6 if you count animated one) but as with every series they all had good points and bad.
ST:TOS - Some of the stories were weak and partly explained in some of the books about series but Shatner's acting was weakest link in series
ST:TOS Movies - Lots have been said about odd/even movies but I found ST V absolutely terrible
ST:TNG first season had lots of lame episodes and some clunkers in other seasons but Wesley ones were big ones
ST:TNG movies - I liked them a lot more than most it appears except I really thought last one was misdone
ST:DS9 Agree first season was weak, it got stronger and stronger as it became more serial rather than episdsodal; Disliked end including killing of Dax because actress left (reminds me on TNG and Yarr)
ST:Vgr Least favorite of series especially whole Hologram rights
ST Ent: Liked cast pick and even T'Pol after a while but think they tried to go to far away from continuity too fast. Liked Andorians and Dr Phlox best
ST reboot: Not really sure what they are trying to do - get some movies and destroy everything before? DC comics did same thing with reboot trying to hook in new audience but they lose audience they spent decades building.