Risa

I'd argue she was actually more logical and rational than her former shipmates despite being more emotional. Everything she said in her first episode was completely right, even though it often violated protocol. Logically, either the protocol should be reexamined or T'Lyn should be given more leeway during her off hours. Punishing T'Lyn rather than working out something that would be beneficial to everyone was illogical and irrational. To me, it highlighted the big flaw of Vulcan culture - that their dogmatic and unquestioning adherence to Surak's teachings is, paradoxically, illogical. Spock eventually understood this, as his line, "Logic is the beginning of wisdom, not the end," demonstrates. That T'Lyn quoted that line would indicate that she has been studying Spock and is likely following a philosophy similar to the one he arrived at in his old age. Logic is a tool, a means to an end, but it is not the end itself. Those who fixate on being logical as an end unto itself ultimately have no goal and are often unable to see the forest for the trees.