I like them all, but relative to each other, I prefer S1 and S2 by a wide margin. It's not even close.
S1 and S2 had a firm link to my favorite Trek era, which is the 23rd Century. It did some interesting (and admittedly uneven / controversial) exploring of that era and legacy characters that I thought was fun and engaging...even if it was at times flawed or uneven. But, most importantly, it was DIFFERENT in tone and presentation from the bulk of other Star Trek shows, which is what I appreciated most. It's one of the reasons DS9 is my top non-TOS show...it grew the franchise into a different and unexplored direction (in terms of setting, themes, tone, format...everything).
DSC S3 and S4 have become sort of more generic "Star Trek," except in a setting so far removed from the rest of the universe (the 32nd Century or whenever the heck it takes place), that it's really lost a lot of its punch.
Additionally, the supporting characters and actors from S1 and S2 were vastly superior to S3 and S4.
S1 and S2 had Lorca, Pike, Spock, Number One, Tyler/Voq, L'Rell, Georgiou, and Cornwell in terms of characters I found fascinating and engaging. It had a good focus and arc for Culber who, aside from being one of my favorite characters and actors, seemingly has less interesting stuff to do even though he is now "main cast" and not a supporting player,
S3 and S4 has Book and Tarka. That's about it. Osyraa was a dull antagonist (I actually found Leland and "Control" more compelling). Adira is a poor Tilly replacement who is painfully one-dimensional, especially given the potential with this character. Gray was also a complete bust-- just a waste of a good idea for a character. I love Admiral Vance, but he's be reduced to a pop-in character in favor of the far less-interesting Federation President.
Also, S1 and S2 seemed to have more organic things for the main characters to do. Saru had a true arc. Stamets had a very clear purpose and arc. Tilly was well-developed and a great foil for Burnham. In S3 and S4, those characters seem to be less-and-less organic in terms of their utility and involvement in the story. Saru is kind of a generic "alien first officer guy" now. Stamets has lost his prickly nature and is no longer nearly as integral as the mushroom expert, so he's just a generic "science technobabble problem solver guy." Culber has become Troi, except without the empathic powers. And, as much as it pains me to say, the "coworker therapy" others have criticized just doesn't work for me either. It's over-done and it seems forced and (again, here's that word / phrase) not organic to the story. And, Tilly is GONE...with no fanfare...which is a HUGE loss to the chemistry of that group.
So...yeah...I miss S1 and S2 a lot. Yes, they had warts, but everything about it was exciting and gripping. S3 and S4 are fun and I still watch it excitedly every week...but the excitement level is down.