A forum is a place for discussion, but mostly it’s the place to get your questions answered. There are some very good forums for Power BI with a lot of activity and a high chance your question will get answered quickly.
Below a list of some of the best Power BI forums out there.
Microsoft’s official Power BI forum is very active. It has over 12.000 threads and 70.000 posts; a great resource to find answers!
The Power BI section of the TechNet Forum has over 8.000 threads and more than 40.000 posts. Another great place to get questions answered.
The forum has over a 1.000 threads and over 5.000 posts, mostly about PowerPivot.
Stack Overflow has over a 1.000 threads tagged Power BI with over 3000 posts in total. Not all questions get answered quickly from what I see; but according to Imke Feldmann at The Biccountant it does contain high quality Power Query M solutions. It also has useful tags for Power BI, Power Pivot, Power Query, DAX, M, etc.
Over 2.000 threads, but the amount of replies on these threads is not that high.