Hard to answer. Each has its own flavour, and there are strengths in all of them.
First off, TOS is (and isn’t) a lot of things, but one thing it was, was a courageous effort to create something new, a new narrative, a new picture of a future, new visions for what humans can be and how they can behave. That in itself is bold and I respect that, and appreciate it. The tone set by that series continues to shape the series we enjoy today.
TNG, VOY, and DS9 - I like all of those, each for different reasons. TNG flies on the strength of the writing (courageous and asking important questions, which set the stage for dramatic stories) and on Patrick Stewart’s acting (phenomenal - a brilliant and compelling actor: that line, “There. Are. Four. Lights.” gives me chills every time - and there are many others). VOY is a crew like no other, a really brilliant set of characters in unique and challenging circumstances. DS9 asks social and political and relationship and spiritual questions that can’t be asked in any of the other series (because of the setting), and that lets them explore more of the strategic and metaphysical picture of the Trek universe, which I liked.
The new series have a lot going for them too. Strange New Worlds is a fun watch with great stories, great acting, and a steady, mostly episodic rhythm; I am liking it so far. The Picard series I am enjoying for how it develops the stories of these familiar characters. It is a bit of a reminiscing / redeveloping series, developing settings and characters I’m already familiar with, but I like it nevertheless.
The Discovery series is one that took me a long time to really “get” - those first two seasons were a mind trip at times - but now that I’m farther in, and the timeline and setting has settled, I’m finding it’s a refreshing journey in new, uncharted space. I’m enjoying it. I’m particularly enjoying the character of Michael Burnham. She’s got an understated, sometimes steely, always vibrant energy to her, and is a keen observer and communicator. Brilliant character and a lot of fun to see how she engages. She surprises me - I can’t easily predict what she will choose - and that is refreshing.