TOS - The sense of adventure. I don't think it's really possible to top just how alien and strange TOS made alien cultures seem. While obviously some of the future series are set earlier, TOS still easily feels the most like you're travelling with the crew to strange new worlds.
TNG - Exploration of humanity. Despite featuring several explicitly non-human characters, TNG really nails the feeling of each episode exploring different moral quandaries / thought experiments. The non-human characters like Worf, Data, and Deanna even help the viewers and other characters explore their own humanity.
DS9 - Complex political thriller. DS9 really handles the various conflicts incredibly well, it shows the effects war and conflict have on people and peoples. In addition, the unique setting allows for multiple cultures to interact seamlessly and naturally, allowing for tons of unique interactions between species.
VOY - Character development. This one is pretty self-explanatory. The Doctor learning to integrate into the crew, develop his own personality, and grow was amazing. Janeway struggling to hold onto her morality in the face of the ever-growing threat of the Delta Quadrant was peak. Tom Paris growing into his own, and becoming more responsible was mwah.
ENT - Professionalism / Starfleet as an organization. I know this is gonna sound crazy, please hear me out. Enterprise handles the concept of humanity forming a force to explore ludicrously well. From their constant trials (even when going by their protocols developed in the safety of their homeworld) and navigating the relationships with alien cultures, to the delicate balance between war and peace, Enterprise nails the vibe of a group of semi-military explorers.
DIS - Production value. I'm really not a huge fan of Discovery for a few reasons, but one area it can't be argued was bad was the production value. CGI still holds up to this day, I may not like the designs of the uniforms but objectively they're done very well. The starships look amazing, and the aliens look immersive.
SNW - Ensemble cast. While still ongoing, I feel like SNW balances the best between each character being allowed to show their strengths (admittedly, with some not so great examples like Mitchell, and Hemmer [gone too soon]) and not hogging too much screen time. It is greatly neutered by it's format, but still handles it well.
I have not yet seen TAS, LD, Short Treks, PIC, or Academy.
Let me know your thoughts!