I personally am a huge fan of good solid(if somewhat to heavy) gear less hub motors, that can be laced into any size wheel and you never have to worry about breaking a chain or snapping a gear. Quiet, stealthy and you have a massive amount of variability. a crystalyte 5 series can be used anywhere between 500 watts and 5000 watts and still hit peak efficiency. They don't say in that webpage what material the bike is made of, I assume aluminum so i should say anything over about 500 watts you will need torque arms even if you put it on the back wheel.
What kind of price range are you looking for and what kind of performance are you looking to achieve? You can go from what amounts to a light motorcycle down to a slow cruiser that requires you to pedal to get up the smallest of hills. Also a note on internal hub gears, I talked to the guys at my LBS about them and it was suggested that they wouldn't stand up to the strain of a motor, certainly this could be BS and I think ive seen a few people praising how reliable their hub gears are. It probably depends on the quality of the hub.
What kind of range are you hoping for and do you want to pedal primarily, or only as a booster when you want to?