"The Ascent" is also one of my favorite DS9 episodes. I love the bickering between Quark and Odo throughout the whole episode, and especially the ending -- when they tell each other they hate each other, and then just start laughing. They have such a bizarre relationship. It's one of the things I love most about DS9.
The thing that makes Quark so great is that he's one of the few Star Trek characters who isn't, well... a goody-two-shoes, for lack of a better term. Almost every regular character in the Star Trek universe is a member of Starfleet, which means they're expected to live up to a higher standard. As a result, they can sometimes come off as a little too
perfect. But Quark is a civilian, and he couldn't care less about Starfleet's moral code. He's just out to make money and have a little fun.
And he's sarcastic and hilarious, which are also points in his favor.