I stay 2 nights in cologne at arts for business, and they have all what you need, if what you're looking for is a decent and design room.
The hotel is not downtown located, but center is reachable easily with sbahn.
If you can ask for a room facing the canal side and not the ones on the street.
Personal at reception desk was not the most friendly receptionist I ever talked to, but this is common in Germany hotels I guess!
Would come back for sure!
- Free Wifi