TMP is the most cerebral imo and was abandoned in favour of what we have now possibly because it did not actually resemble TOS! Specifically the action/adventure !
I think TFF also explores interesting themes and can be very thought provoking if you want it to be. And judging by the reaction to TMP and TFF, it doesn't sell well. TUC also deals with interesting dilemmas.
INS and NEM try to be thought provoking, but fail miserably.