Mm, I like them both for different reasons.
Adachi is just an entertaining asshole and he's loads of fun to watch when he does his thing. He's not really all that "deep" of a character in my opinion, at least by Persona standards, but that's okay because he doesn't /need/ to be.
Akechi on the other hand is far more complex of a character, IMO. He works much better as a rival and foil to the MC than Adachi was to P4's MC.
He and the P5 MC have a tense relationship for many reasons. At first it just seems like Akechi's detective role is the source of tension.
But eventually (and in hindsight) it becomes clear that Akechi desperately wants what the MC has. He just sees no feasible way to obtain it. So, it's nice for his character that he gets extended a sympathetic hand despite his actions.
I do have to say Akechi could use a few pointers from Adachi on his villain theatrics, though. Adachi is a lot more sinister.