What kind of pen is best for lefties?
Left-handed writers tend to hold their pens at a relatively steep angle, nearly perpendicular to the paper (some righties do this as well, and the following comments apply to them too). Most nibs are shaped for use at a shallower angle, and can catch and skip when used by a lefty.
In general, the nibs most suitable for lefties are rigid with rounded tips. Parker Duofolds nibs are a good bet, as are those found on Sheaffer pens from the later '20s and '30s. Many Sheaffers are found with turn-up nibs, which are especially well-suited to lefties. Manifold nibs, designed for making carbon copies, are also a good choice; these special-purpose nibs were offered by many companies, though Waterman nibs are the most often found so marked. Reverse oblique nibs, with a chisel tip sloping back to the right, are supposedly aimed at left-handers, but most lefties I know who use calligraphic tips prefer a simply straight-cut stub or italic.
Most left-handers don't do well with flexible nibs. When a flex nib is held at a steep angle, the writing pressure that would normally cause the tines to spread does more to make the tip dig into the paper instead, while the flexibility of the tines magnifies the effect of any roughness encountered. This can be particularly severe when the pen is pushed rather than pulled across the paper.