I have stayed at several hotels in Budapest, as I am often there (Monthly / BiMonthly). The Kempinksi is by far the best hotel I have stayed at in the city.
The customer service, front desk, staff, etc. are EXCELLENT. They are absolutely fantastic.
The rooms are clean, comfortable, and well appointed. Any problems are well taken care of by the staff who go above and beyond.
The restaurants are also nice, Nobu is always a winner. The spa is fantastic and shouldn't be missed.
This is certainly my go to hotel when in Budapest.