The Giants would make sense since Captain Sikso wore that hat. I mean usually i wear my favorite teams hat. I wouldn't wear another team's hat so I think its safe to say its the Giants. And wasn't Worf Willie Mays in an episode too?
That and i think Sisko liked Jackie Robinson for some reason, at least i think he was talking about Ebbits field once, this might have been in a novel however. Maybe Millieniuem trilogy? I think it was. That makes sense he seems to like the best black ball players Willie Mays, Jackie Robinson, Hank Aaron, Adam Jones
"Now, how are we progressing, Mister La Forge?"
"About like you'd expect, sir."
"Splendid. Splendid. Carry on."