Ganandorf is probably my favorite heavyweight character. He's got insane attack power and armor, but he's very slow on his feet. His ground-based physical moves are reasonably fast-firing, have good range, and have a lot of kick. However, beware of his up-A move. While most characters' up-A moves release a fast blow to the air, Ganandorf's unleashes a powerful but incredibly slow kick that deals ground-based area damage. It leaves him completely vulnerable for over two seconds, and it's all too easy to accidentally unleash at a bad time. His up-A smash is his best physical move for getting KO's because it's faster than his right/left-A smash, though it requires a little more aim. His B moves are basically the same as Captain Falcon's, just slower and stronger. His aerial A moves have unusually good range and power. Both his down-A and forward-A aerial moves are meteor smashes, making him excellent for aerial interception and KO's. His up-B move is pretty good for recovery. It allows you to spring up, grab enemies above him, and send them flying, so it's great for aerial interception. Plus, every time you use it on an enemy, you get to use it again while still in midair, so as long as you have enough enemies close by, you can use it as an unlimited recovery move. Ganandorf's down-B move shoots him a good distance obliquely down from the air or horizontally on land; it's good for penetrating through a line of assailants. His right-left B move is similar, though it's faster and can only hurt one enemy at a time. It makes him lunge horizontally (though not as far as his down-B move); if used in the air, it spikes enemies down on contact like a meteor smash, and if used in the ground, it uppercuts them into the air on contact. His B move, like his up-A move, is slow, but ridiculously powerful; use it only if you know how to time it correctly.
Remember: even though many of Ganandorf's moves fire fast, all of them leave him wide open for a long time. Consequently, hold your ground and attempt to send your enemies flying with a single blow. Ganandorf is great for defense because he is a master of both aerial and ground-based interception. If enemies charge you, either intercept them with your right/left-B or down-B move, or give them a left/right-A kick with your long legs before they reach you. If they come at you from above, intercept them with your up-B move or your forward-A aerial meteor smash. Of course, Ganandorf has no projectiles, so you probably won't be able to play defense forever. If you must attack, compensate for his slow running speed either by relying heavily on his down-B and right/left B moves or, as I prefer to do, rely primarily on your aerial A moves. Use Ganandorf for small levels, preferably with platforms. Don't use him to duel insanely fast characters or those with powerful projectiles. Don't become the center of attention in free-for-alls, for Ganandorf is easily overwhelmed by numbers. In team battles, let your partner deal damage and protect you from projectiles while you focus on KO's.