My all time favorite is the OS 40sf. Mine is about 15 years old and has never had a part replaced except for o-rings and gaskets. Fires up on the first flip every time and I do mean everytime. It is an ultra reliable engine and that makes it such a pleasure to run. Still pulls a 4.5lb plane like it did 10 years ago. All other sf engines are some of the best OS has ever produced IMO.

Shane