BTX cases would have the best airflow, because that's what they have been designed for. For some reason the standard hasn't really caught on, though.
Positive vs negative pressure is somewhat interesting to me. I'm going with the Sonata III case, and it seems that Antec usually designs their cases to be negative pressure (PSU and the 1st case fan pushing air out at the top/rear), but positive pressure could have the advantage of less dust being sucked into the case (because the main fan has a dust filter). Which one should I go for?