I'm partial to a powermeter being the best bang for the buck. After that, a very well fitting race kit. After that, race tires and latex tubes. After that, race wheels. All assuming a functional bike that won't break when you stand up to accelerate.

For a tt, I'd say skinsuit first (150-200), aero helmet that fits riding style second (80-200), disk wheel cover (100 bucks) third. That's assuming you have a good position on a functional bike with aero bars.