The RB211 is a pioneering aero engine design and a Trentsetter for the future. Rolls-Royce took a huge risk with implementing the step change in technology of going from a two- to a three-spool design. However, in the end the company was very successful. The Trent has been developed from the RB211 and it is the only three-spool high bypass turbofan in commercial service today. My personal view is that it's three-spool architecture has more development potential for the future than two-spool designs (e.g. consider the amount of variable geometry incorporated in competitor two-spool engines and the pressure ratio of their High Pressure Compressor). Besides, the engine is more compact and lighter, which is beneficial in terms of fuel burn and the additional payload capability of the aircraft. Therefore, the RB211 is my favourite engine and it should be on the list!!!