E-plus
Two years ago I used E-plus (green awnings, all over Germany) for a two week visit from USA.
NOTES
Very helpful. No cost for the sim chip. Helped me activate it in the store.
Seemed to have two pricing plans, one had low cost German calls, high cost international. The other vice-versa. I went with the low cost international.
Was not interested in data, just voice, so can't comment on data rates.