I am partial to the Hori Fighting Commander ($20-$30). It is very ergonomic and has turbo switches for each of the six buttons as well as slow motion mode (which comes in useful for the final boss of Kabuki Itouryodan). The PC Engine Duo-RX pad is a close second for comfort but is extremely expensive (usually around $60) and hard to find. The only functional drawback with this pad is that it is sometimes difficult to do moves that require a diagonal direction.