What bond\crystal has the most regular pattern (i.e. least irregularities)?
In nature you need to look for precious gamestones and then do optical selection (diamante and ruby would be good candidates). The same would be true for synthetics but of course as mentioned by Jon you can also take elements (Si, Ge). General rule: they should be hard (smaller chance that pressure will affect structure, by pressure I mean also your tools) and you need to grow them slowly.

btw it's hard to call graphen crystal pattern -- it's 2D.