Here's how it works for me.
TOS- Can excuse for the era it was created in and laying the groundwork. Does some great swashbuckling space adventures/morality plays.
TNG- Starts out dull but continues the morality plays and excels when it really starts developing the characters and their relationships with one another. Data for instance is arguably a cliche character but is defined by how his quest to be more 'human' results in a likeable, dependable individual.
DS9- Explores what happens when the Federation faces threats both internal and external. Addresses issues of redundancy by giving us a setting where non-Federation races are part of the main cast and has a bit more daring as a result.
VOY- Has a brilliant concept that it absolutely fails to explore. Everything I find boring about TNG is on display here with essentially none of the strengths that made that show work. It's a ship out by itself but never really having to make tough choices or feeling like it's under pressure and as it goes on digs in hard to TNG fanservice to keep viewers.
ENT- Carries over a lot of Voyager's problems but redeems itself somewhat latter when it starts exploring what laying the foundation for the Federation actually means, especially for a humanity that has just crawled out of self inflicted nuclear war and isn't the big deal they are by Kirk and Picard's times.
BEY- Haven't watched.