To me it seems logical that the best-seller 737 is and has been the most widely operated (that is, the most number of different airlines) aircaft. But I dont know for sure. The A32X series is operated by many, many different airlines. Then again, the 757 series is very popular among charter airlines- which really bumps up the number of airlines it is operated by...... Does anybody know for sure?
And what about widebodies, specifically?