I think life is more complex than labelling people as good or evil, but DS9 is art and in most artistic works there are protagonists/heroes vs. villains. Sisko obviously is the hero/protagonist, and Dukat (plus Weyoun and the Female Changeling) is the villain.
But Garak, I'm not sure. I think Garak is the opposite of Dukat in some ways. Dukat could do good things, when needed. He loved his people, loved his daughter, etc. But the bad things, such as overseeing the rape of Bajor, wanting to destroy the Federation and joining with the Dominion, seeking to destroy Sisko via the Pah Wraiths, etc. Garak on the other hand was willing to help the Federation fight against technically his own people, was a good friend to Dr. Bashir, etc. He could kill when needed, but not in a sociopathic or wanton way.
What say you, is Garak evil?
MAINLY: SHADE OF GREY