There's so many ways to level so the question is hard to answer. If you're just doing heart quests and DE, I found GS and scepter/torch to be pretty all around flexible. Hammer's more of a PvP weapon from what I've seen.
I've really enjoyed the Greatsword. Leap in, cast the binding blades, pull all the mobs to you, put your symbol on the ground and spin to win with Whirling Wrath. It's really nice to be able to set up AND finish combos with the Guardian. Not sure if other classes can combo with themselves but it's useful.
I also like Sword/Shield and Sword/Torch depending on if I want to be more defensive or offensive. I like the instant teleport + blind of the second Sword skill.