My answer for all eras/incarnations (even DS9):
That's definitely among my favourite things about Star Trek, and indeed, the Lego Movie :P
TOS: Tons of charm, humour and adventure. I love its 60's look. So many episodes are good solid scifi, and will never age. Spock.
TNG: It was brave enough to be its own thing. From the characters, the ship and the new races, TNG never felt like a rehash, and therefore succeeded on its own terms.
DS9: Story arcs allowing for a richer universe. Very strong ensemble cast. Found its feet quickly.
VOY: Great premise. Superb ship design. The Doctor.
Enterprise: I find the late 21st to 22nd century to be a fascinating time period. It's Star Trek but it's also more familiar. They did a great job of conveying the pioneering days of early interstellar exploration. Inferior technology, inexperienced crew, the first two seasons are great fun. The NX-01, whilst not being what I was expecting is a superb design inside and out. The fourth season is awesome.
DSC: Has opened up Trek to new possibilities. The universe seems more alien and strange. I love the characters, who, whilst flawed, still strive toward the Star Trek ideal, despite being in an era that is dealing with a brutal war. That characters like Burnham can fall so far and still make speeches about trying to live up to the starfleet code makes her that much more compelling - as with DS9, it's easy to be a saint in paradise. The USS Discovery design manages the seemingly impossible task of making the simple geometric shapes of the TOS era work for a 21st century audiences. I'm shocked that it was apparently designed by committee. Saru can never have too much screen time. Mirror Universe!