Well, in general in Germany I am recommending the youth hostels of course and then there are the Zimmers. But in case you are in a business trip or want just to enjoy yourself, you can go to one of the Hiltons in Mainz.
I stayed here several times. It's kind of pricey, but if you have the money (or someone else is paying), it's a nice place. The service is excellent. The rooms are clean and large. The restaurant in the hotel is quite expensive, but there are lots of alternatives right nearby.
It's a good location if you have a car... very easy to get in and out of. If you're on the right side of the building, you get a nice view of the Rhine. Also, a few times a year there are some festivals right nearby
There were a couple of small hiccups with my stay here: there was work going on somewhere so I had to walk over the bridge from the foyer to get to the room, and they'd made a mistake of some sort with the reservation. BUT the room was excellent (enormous, spotlessly clean, a little characterless, maybe, but I think you'd expect that too) and breakfast was stupendous - huge, and taken in a dining room with a vast wall-window looking out over the Rhine, where barges were slowly passing in the snow. WHAT'S MORE, it cost me less than 50 Euros ! Hilton have special offers at weekends.