I know whats up with this. The orthodox advice with beef would be a cabernet. You will want a tannic and smoky flavored one. Other posters have recommended very good varietals, but for a flank steak, 90% of sommeliers are going to recommend the same thing I am.
An affordable option that would be a good match is Casa Lapostalle cabernet. Montes also has the flavor you are looking for. You could do well with some of the more austere california cabernets, but I think they have too much fruit.