For me it's along the lines of
set a budget then...
First, it fits your body and your riding style(i.e. slack or steep).
second, It's fast.
I think the first should be first on anybodys list. Only problem is it requires a good fitter if you've never ridden a tri bike before.
For me, money no question, I would be on a P3C or Felt B2 carbon, with a stack under 50 and a reach over 41 it's about as good as a bike would fit me. For comparisons sake however, a Kuota Kalibur also meets the above stack and reach parameters but I think the tubes on it are quite chunky in comparison to the P3C or B2.
I don't believe carbon is the end all, be all of bike design. I would gladly take a Cervelo P2sl over almost any other bike and I would simply need to run a negative rise stem.