To be honest any 4 stroke in the 60-75hp will be good. Mariner, Honda, Suzuki etc - Out of these three the Honda and Suzi will be the smoothest and quitest. I had a 60hp Mariner on mine, and whilst it was fine, the 75hp is the optimum engine size IMO.
BUT! My overall choice nowadays would be the 75hp Evinrude Etec - awesome, engines, slightly noiser that the 4 strokes, but lighter, smaller and bags of power.
A Warrior 165 with a 75hp Etec on is THE package to get.