For a lack of alternatives I am with Telekom. I could get 250MBit/s but I am still on a 50Mbit/s contract. However I have noticed that I’m regularly exceeding the promised 50Mbit/s by 5-10Mbit/s so I get a little more than promised.
As others have mentioned Vodafone has been pretty good over the last years. Can’t really go wrong with them. Mobile contracts are a whole other matter in Germany. I’m with Drillisch but I heard in a lot of areas the coverage isn’t great with them. Personally I haven’t had much trouble in that regard though.