I have had more than I care to remember but my favorite was a Werks B6. That engine had tons of power and an awesome top end. Was easy to tune and would idle through an entire tank of fuel without bogging down ans stalling. Second faovorite was a OS .21 VZB V-spec. It was another great engine Very smooth and responsive wasn't as easy to tune as the Werks but it was pretty good too. My favorite RTR engine is the teal Headed Mach 427. Very reliable engine good power and topend for "cheap" engine.