The definition of a pro-level tool of course depends on the application domain. For me, this is programming, data analysis, writing, teaching and all the other things that I need to do as a scientist. The most important criterion for me is the flexibility on the one side, and convenience on the other one. I need a fast machine (to efficiently prototype analyses), but it also has to be maximally portable, with great battery life (since I am constantly on the move), it has to support advanced workflows efficiency and offer great automation.
Currently, there is no laptop on the market that suits my needs better than the 15" MBP. It comes with fastest CPUs Intel currently offers, it has fairly decent GPU for numerical applications (or occasional gaming), its very convenient for working with texts due to great display.It offers the most flexible and convenient connectivity of any device I ever worked with: I can connect all my desktop peripherals, including a display, the power source, and the external backup disk with a single cable. And at the same time, it is ridiculously light, fits into a small messenger bag and at the same time easily offers a full day working time on battery. And of course, it runs unix, which gives me the automation tools I need.
P.S. If Apple wouldn't make the 15" MBP, my second choice would probably be the Precision 5000. It offers comparable performance in the CPU/storage department, but it would come with a significant hit to portability (due to weaker battery life).