I voted Tuck. Great leadership qualities, ability to keep things under control, an offensive force inside and outside.
Not Stewie, because "difficulty" isn't the right word. Impossibility is more accurate. She's just too unique to be merely difficult to replace. Uber-athletic 6'4," freakish wingspan, dominant in every category of performance. A one and done package. (With some similarities here and there, Lou will carve her own path, not replace in any real sense.)
Not Moriah, because Danger, despite some dissimilarities as well as significant similarities, is quite intriguing as a potential replacement.