I''f I was to judge who was best at acting as "not human" I would go with Data. He does have some strange quirks at times like taping his finger on the console when talking about being nervous in "The Defector" or his slow reaction to Picard's painting in the episode were RIker is accused of killing a guy and the space station blows up. For the most part though he really feels like someone without emotion. He really does look at things simply from a logical point of view. His attempts to be more human almost feel like he is simply mimicking human behavior in hopes it will make sense to him on some kind of logical perspective. Every once in awhile I think he gets close to getting that perspective such as his friendship with Geordi and Lal but he never makes it all the way of course.
Spock on the other hand is very human. He is simply someone who is smart and kind of arrogant because he knows he is the smartest guy in the room at all times. I think he has tons of confidence and likes Kirk and McCoy because they are great at their jobs and offer him a challenge as people who come close to keeping up with him. Plus they respect him and I think he is someone who rather have someone who respects him than someone who might get all emotional over him. Plus he see's them as people worthy of his respect.
Odo is basically someone I see as being very sensitve and emotionally vunerable but acts tough and rigid so people won't see his true feelings. He doesn't trust people or society that much because people and society haven't ever done much for him. I think he likes Quark so much because he kind of wishes he was more like him. Life of the party and and says whatever is on his mind and doesn't care what others think for the most part. KIra I think he loves because she is very much like him. Bad life up until DS9, both emotionally vunerable both feel the need to be strong and act tough. They are kindred spirits.
Jason
I actually see Odo a lot like you describe Data (which I totally see and pretty much agree with), where all the human bits that are not emotional personal stuff (which he is still an emotional being, he feels love and loyalty etc) are a facade he presents for other people, while not at all understanding it. I mean even on a basic level, he assumes a humanoid shape, he could actually go for something different, tentacles maybe. I think a lot of the distance he keeps is based on that... like he's trying to keep an observer's perspective on a subject (though I also see his emotional stuff in there too). He often "lumps" humanoids together, regardless of species, because he sees their similarities so much more than most, esp compared to himself. I especially liked early on when he didn't (appear?) to have quarters, just his "pail" and seemed perfectly fine with that and other things that were particular to him and being a changeling. then later on when he's taken to assuming different shapes on his off hours, to explore "being" other things. Like he's not particularly interested in being any more humanoid than he already is, which is a bit different than Data (and even Spock, he grew to appreciate a lot of "human" qualities, and was happy to adopt them).