I want to say Ravenclaw, because he is an intellectual, however, his kinship with Kirk (clearly a Gryffindor) indicates a strong sense of loyalty and justice. While he is industrious, I do not see him as a Puff, because he has a great deal of pride in himself. nor is he as devious or ambitious as is required by a Slytherin.

I'm going with Gryffindor, as I do believe, more often than not, he does value a sense of right and just over a sense of logic. Plus his feelings for Kirk, Uhura and the crew tend to overwhelm his logic.