Depends on the brand of motor. The following applies to motors with 80lbs or more of thrust. I've used plenty of lower thrust motors but haven't experimented with different props.
Minn Kota has a very good 2 blade prop that come stock on the motor. It's weedless, quiet, durable....the only downside is it's fairly large in diameter, so you'll bottom out slightly sooner. Not really a consideration for most.
Motorguide has a metal 3 blade that has good characteristics, but I'd only recommend it for a 109 lb motor... and you can damage your motor if you slam it into solid objects. I didn't care for the Safari 2 blade, but they may have improved the design since I first tried it.
The best prop I've found for Motorguide is the Kipawa weedless. It's very weedless, quiet, and has nearly the same top end you'll get out of the metal 3 blade. Motorguide sold this prop under the name Hydrilla Hacker, but evidently, they're no longer buying the rights to sell it. You can still get it from the manufacturer:
http://www.kipawapropellers.com/