I would love to meet the person who did not consider themselves a realist. Excluding that option, however, I have to say I'm pessimistic in particular and optimistic in general.
When I am called upon to make a quantitative guess, my natural tendency is to exaggerate the odds of the worst plausible outcome. When I am called upon to complete a task, my natural tendency is to exaggerate the difficulty of completion to the point of paralyzation. And when I am called upon to describe what I have accomplished, my natural tendency is to understate my claim as much as possible.
On the other hand, I honestly expect things to turn out for the better. I think the world is a fine place, that a century ago it was worse, and that a century from now it will be better. I am appalled at those who speak of needing to compromise their principles - "have they no faith in the power of Good?", I think. When I meet a friend coming out my front door, he gives me a ride to the Metro just in time to catch the train, and both transfers come within the minute, I am not even perturbed - I just look at my watch and say, "Hey, I'm almost on time today!"
All that said, though, I reject the words
, let alone the trichotomy. They are useful ways to describe the inaccuracy of estimates (a la "You don't think a month to finish the book is optimistic?"), but dispositions don't divide along those lines.