OmniFocus - because of its version of start dates (essential for the types of tasks I have), its flexibility and its "hinterland" of learning materials, AppleScripts and third-party inter-operative apps. Its latest UI I do not love, and I'm still waiting for it to be sync-able with OmniPlan (which seems a no-brainer to me), but in view of its other virtues, I'll be patient.