I'm finding it difficult to answer this (and the Voyager one). What I can say is that even the most alien characters had human features in Trek. In fact, most of the aliens are portrayed as having some human aspect, but magnified or amplified beyond what we normally would expect -- the Klingon anger and warlike ethos, the Ferengi greed, the Romulan inscrutability, the Vulcan logic, etc. This is IMHO why we can relate to Trek aliens so well. I reality, I don't expect we would be able to fathom real aliens' motives.
Data's childlike aspect is particulary appealing. When Riker first meets him in 'Encounter at Farpoint' there is even a reference to 'Pinoccio' thrown in there. I think I will go for Data...