Rimowa [in Frankfurt, LH M&M Worldshop(s) and related topics merged]
Not sure if mentioned before but a great and convenient option is the FGS shop in baggage delivery hall B. They also provide 25% to travel industry staff, will give you the full 19% VAT refund, and it saves the trip to Kelsterbach. Their selection/space is limited but the turnover is high so it's worth checking out.