Hondas and Yamis seem to have the best reputation, but also the highest costs. If I am not mistaken (I often am :) ) Suzuki make all the four strokes that BRP sells under their Johnson brand. I think most will be good motors but you should look into who in you area will service them and what kind of costs are involved in each brand.

My father-in-law has a 15 HP Honda which he loves but he says it can be costly to do the regular maintenance. My Fishing partner has a Merc 25 HP Four on the back of his boat and he needed to have warranty work done on the motor twice since he has owned it (they totally replaced the power-head the second time)He may have just got a bad motor but it has soured him on Mercs.