The most dominant engine of that era is arguably the BMW M12/M13 motor. What makes this engine special is not just the insane horsepower or championships it is that it was based on a production M10 four-cylinder iron block. This block first entered production in 1962 and would continue to be produced until the late 80's. Even after turbo motors were banned from F1 in 1989 the M10 block would still be used by various teams in various Motorsports to great success. That is what you call a legendary engine design and one of the many reasons BMW has earned the reputation as one of the world's best engine builders.
The M12/M13 is the most powerful four-cylinder ever produced by BMW. How much peak power it actually hit we will never know as there was no accurate way to measure the output. Not to mention boost was pushed to whatever level teams thought they could get away with in qualifying so rumors of 1800+ horsepower in qualifying trim are not far from the truth. Whatever the power output actually was this little 1.5 liter saw outputs ranging from 740 horsepower to 1500+ throughout its years in Formula 1.
The rpm achieved was 11,500. The new F1 motors for the 2014 season rev to 15,000 rpm. They also offer up a similar displacement at 1.6 liters although from six cylinders instead of four. They also are direct injected versus the port fuel injection of the 80's. Boost is limited to about 3.5 bar (50.7 psi) by the fuel flow rate for 2014 although there is not an official boost cap. The 80's M12/M13 saw over 5.5 BAR (79.7 psi) in qualifying and boost had to be capped at 4 BAR (58 psi) in 1987 due to fears power was getting out of control.
How much horsepower the new Formula 1 motors put out will have to be seen but it is safe to say with regulations limiting boost, fuel amounts, and fuel flow rates the new engines will not even approach the power levels seen in the 80's. When BMW was allowed to run wild they made so much power the motors ended up being banned from Formula 1 competition. That is perhaps the greatest compliment to the M12/M13 engine's prowess as any.