I think Floyd has a great chance of remaining undefeated with or without Pacquiao. There currently doesn't seem like there's anyone out there that can stylistically outbox him. There are a few fighters out there that can definitely hurt him, but I don't they can knock him out or outbox and win on points which may have to happen in order to beat him.
His contemporaries like Pacquiao and Martinez have the best chances. Or a young, fresh and explosive contender on the rise. But they all would have to keep the pressure on from start to finish and bring the fight, or call it a night. Regardless, it still seems like Floyd is a technician with the edge and the winning formula that no one has been able to figure out yet.
However, he's 35 now. If he's stays in too long and gives someone fresher or a contemporary who has maintained well a shot, he can be upended eventually. Roy Jones was also once invincible. Floyd is stylistically and defensively that good to stay ahead of the field, but every boxer has there time in which they should get out.