Mc-Coy MC-9 (also sold by H9) has worked the best for me by far. Use FAI 0%-nitro if you can... it hates nitro. Otherwise, you may need to shim the head a bit to keep from burning those expensive plugs too quickly.

My engine seems perfectly happy running Wildcat "Jet-A" (5% nitro, 23% oil, designed for the ducted-fan crowd and the RPM's we're turning), but my friend Carl's is burning up plugs too fast. His engine is newer, has the newer style pipe, and seems a bit less happy with nitro than my engine. We have some head shims to install on his (got them from Morris Hobbies), but haven't done it yet.