Bold Hotel Frankfurt is a very well renovated apart-style hotel located in the suburbs 5 minutes away from an S-bahn and tram system.
The single studio that I stayed it was absolutely excellent as it came with a very modern design and quite spacious, coming with an extra bonus of the studio kitchen. (the pictures actually don't do it justice, the room was bigger than my student accommodations!)
One big gripe one would argue is the lack of air conditioning, which might be a must in the summer for some, however the hotel remedied this with a new table fan that can be pointed straight at yourself right next to the single bed. Guaranteeing you to actually be quite comfortable.
Considering the price, for me it was 30 pounds a day, this place completely exceeded expectations - and arguably it could be even nicer in the winter as the room would be naturally cold - I highly recommend this stay if you're looking for a value apartment hotel in Frankfurt.