Shaquille O'Neal was known for double-handing the ball into the goal so forcefully, the backboard would shatter. While rim technology makes that not particularly possible any more, it's still a powerful and demoralizing dunk to pull off. You need a really high vertical to pull of a two-handed dunk. Practice standing under the rim and jumping straight up until you can touch your wrists to the rim. Suggesting you're so high you could dunk it twice, in the double-pump dunk you bring the ball back down to chest level at the apex of your leap, then force it back up to slam it with authority. Some notable players, Tracy McGrady among them, would do this regularly while spinning in the air, doing a 360 dunk variation. As you approach, bring the ball into your abdomen and back, extending your arm behind your body and up in a circular fashion, like a windmill spinning. At the apex of your jump, bring your arm all the way around to throw it down like a boss. Dominique Wilkins, the Dunkmaster General of the 90s, used to blow crowds away with this spectacular dunk. Either two-handed or one-handed, the tomahawk dunk involves bringing the ball back over your head by bending your elbows and slamming it forcefully into the hoop, as if you were chopping a tomahawk. "Dr. J" Julius Erving popularized this poster dunk, as well as Darryl Dawkins, who broke several backboards tomahawk dunking. While he wasn't the first player to complete it, Vince Carter wowed crowds at the 2000 NBA dunk contest by passing the ball under one leg while in the air and slamming it with authority. It didn't hurt that his forehead was almost touching the rim. If you've worked your ups to that height, try passing it under one leg and dunking it.

Summary:
Learn the two-handed slam. Add some flair with a double-pump. Turn on the windmill. Chop the tomahawk. Go between the legs.