Just avoid the 777 because you never know if a config switch will happen. I would pick between 763 and 747 only based on schedule convenience, the seat and service is the same on the two aircrafts.

If you are allowed to select a LH flight instead, you might also consider that as a good alternative for a FRA-ORD flight. Great First Class terminal in FRA, (much) better food and wines, both on the ground and on board, no consensus on seat comfort for sleeping for LH vs UA (search for other threads), but it is a day flight... IFE is most likely a win for UA (but no experiences with refurbished LH first).