score:9
I can understand your question. I may not agree with John Piper all the time, but when I would like a 21st century Reformed position on an issue or a Bible verse interpretation, especially when there are multiple viewpoints even within the Reformed camp (as you know, there are variants such as Hyper-Calvinism, Neo-Calvinism, Christian Reconstructionism, etc.), John Piper is usually my initial go-to source to get an even-handed Reformed perspective before going deeper or broader.
His extensive Desiring God Ministries web resources which include Articles, Sermon collection organized by Scripture or topics, Podcasts, In depth exegesis, Books by him and others, etc., prove to be:
(Note: I'm only endorsing the web resources authored personally by John Piper, as some articles / sermons in that website don't rise to the same level of quality)
Other reasons for his popularity (from @curiousdannii):