I hadn't read this thread before and came in to see if 10 wickets in an innings is mentioned. It seems most difficult achievement in Cricket but 6 sixes in an over has been tougher if we consider first class matches too, not just internationals. Otherwise test cricket has 2 instances of 10 wickets in an innings while 6 sixes in an over happened twice as well in international matches(once ODI, once T20). However, if we consider first class matches, 6 sixes in an over happened only 5 times whereas look at number of times 10 wickets in an innings has happened in first class Cricket!
http://stats.espncricinfo.com/ci/content/records/209288.html
I was very surprised on finding it out first time.