I spent 3 nights at the Melia Berlin last month.
It's a lovely hotel and pretty close to being 5 stars. The room was spacious with a very elegant style. It had a king size bed with really nice sheets and duvet. The extra charge for WiFi was quite high, so try to get a rate that includes it. Same goes for breakfast -- get a rate with breakfast or eat outside the hotel.
I was there on a business trip, but the location right on the river was so convenient I was able to do a bit of sight-seeing each day too. It's maybe a 10 minute walk to Brandenburg Gate and Museum Island.