DS9 was a near-perfect ensemble of great characters working together, so I can't really say "best". I will say a personal favorite is Nog, who should at least get the "most improved" award. In seven years he went from an illiterate pickpocket to a mature engineering lieutenant, so yes - I'll go with Nog.
Nog didn't have much depth, but his trials were understandable and desire to excel genuine. My son, 7 years old, who has just started liking the show, really relates to him.