Most students with aspirations of a pursuing a PhD in pure mathematics at a top grad school takes the "standard" curriculum as an undergrad which includes 2x algebra, 2x analysis, 1x complex analysis, 1x topology, 1x linear algebra, 1x probability, and 1x combinatorics.
As a rising junior who needs to figure out what to do over the next two years and knows he wants to go to grad school but doesn't have the faintest clue in what particular field:
What are the most important "nonstandard" undergraduate math courses for pure math grad school?
Note: I asked this question on math.se but it was put on hold there. Hopefully this question is better suited for this site.