He's a solid P4P all time #1 candidate especially if he fights Floyd (The result is rather obvious). There are things he accomplished that has never been accomplished in history and very unlikely to be repeated in a foreseeable future.
I wouldn't know where to rank him, really, but Pacquiao has surely been the best we've had for some time. When I think back over the past thirty years or so, the real stand out fighters, for me personally, have been Duran, Chavez, Pacquiao. Pacquiao may fall a little behind Duran and Chavez, But I do see him in that kind of class. With his resume and achievements I expect he'll be ranked pretty high at the end of the day.