Like many others I don't care much for the word 'rationalist', since it comes with lots of preexisting negative connotations. I think we're most likely stuck with the term, but to amuse myself, I came up a list of alternative terms. The Seattle LW group enjoyed the word 'cognomancy'.

I don't claim any of these are original to me; I know I've lifted a couple of them.

cognomancer
optimancer
orthomancer
ratiomancer
logimancer
utilimancer
mentamancer
metamancer
economancer
psychomancer
bayesian empiramancer (really you could add 'bayesian to most of these')
mathamancer
cognitive engineer
optimization engineer
neuromancer
cognologist
cognician
optimician
utilitician
ratician
orthocognician
economician
mentician
empirician

I really like the suffix -mancy; it's really fun to think of yourself as doing any kind of -mancy.

I'd love to hear other funny alternatives to 'rationalist' and other related terms.