From the outside, it is hard to assign credit to certain executives for Apple's success. I have thought of Serlet, though, as a bit of an unsung hero behind Jobs, Ive, Schiller, and now even Cook.
Since Serlet came from NeXT, I suspect he had a significant role in the transition of the Mac to OS X (which to me is almost better described as the application of Mac UX ideas to the NeXT OS).
At a minimum, for Apple to have ended up with a MacOS that ended up being adaptable to phones, tablets, and even set-top boxes, Serlet must have had the wisdom to choose the the right architects and technical leads and let them run. That alone is an executive skill that is incredibly rare.
Hopefully, Craig Federighi can continue the impressive track record of Tevanian and Serlet.