More emphatic term for "Expert"?

The most obvious short word I can think of that could be seen as being a level ‘above’ expert would be guru, in the third sense given by Merriam-Webster in the link:

a person who has a lot of experience in or knowledge about a particular subject

Have you considered using the Dreyfus model of skill acquisition? In particular, his ranking is:

1. Novice
2. Advanced beginner
3. Competent
4. Proficient
5. Expert

Note that each level has a specific, quotable, empirically-driven definition that allows one to (more-or-less) unambiguously identify which category one belongs to.

That might be helpful if your application is a self-survey which is otherwise vulnerable to differences of interpretation: a standard model has the benefit of normalization (to a larger extent than, say, “on a scale of one to ten. . . .”).

That said, since perfect standardization is an impossible ideal, if you implement this, you might consider offering an "other" option.

Alternatively, the word specialist comes to mind.

If the level being described is the ultimate attainable, perhaps the term paragon might suit

A person or thing regarded as a perfect example of a particular quality [ODO]

Similarly, you might consider apotheosis (although less often applied to an individual than to her accomplishments), paradigm, or epitome.

Perhaps savant:

a learned person, especially a distinguished scientist.

In my experience, this word has connotations of almost supernatural expertise.