I am looking for companies that offer prepaid/postpaid SIM card with good data plans that I can use with my phone to make WhatsApp/Skype audio/video calls.
I have read that some providers throttle calls under a data plan.
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German telecom providers usually don't have very good data plans. I can suggest Premium Sim as a cheap option. 10 Eur/month for 1 GB LTE (not always LTE but yeah) or 25 Eur/month for 5 GB. (Both connections without a contract, so you're billed monthly and can cancel your connection anytime). You will also get unlimited free calls to all networks in Germany. Haven't heard of throttling here in Germany.
You can check http://prepaid-data-sim-card.wikia.com/wiki/Germany Their information, in general, always correct.