For me the weakest cast as far as riveting characters go is TNG and always will be. So much blah.
I adore my VOY but cast wise DS9 wins by light years, also has far more characters.
I have some unusual ideas on the cast I think. Firstly my favorite cast is not necessarily the best cast. That would be STNG. It's strength may be chemistry as well as two standout stars with Spiner and Stewart, easily the best captain of Trek.
Best cast from top to bottom, surprisingly to some probably is Enterprise, it's solid throughout, without any one character really standing out save maybe Archer because of what he represents in history or Trip, because he is likable (I normally am not a huge fan of southern characters either). There isn't one I dislike and they all have their moments on the show.
Deep Space Nine is second. Another strong cast, with probably the best female character in all of Trek with Kira. Odo, O'brien, Sisko all have strong moments.
I have to mention TOS, they have strongly identifiable archetypes, are well known but the secondary characters are not developed enough nor are the actors good enough to stay on screen for any length of time.