It's tough, from a business sense it's clearly John Stones, from a player playing for Everton (in my time) it's one of Arteta, Baines, Lescott, Pienaar, Cahill, Coleman or Jagielka.
Incredible that we got those 7 for £19.06m especially when Niasse and Mori cost 20+ million
...add to that Andy Gray £250k and Kevin Sheedy £100k.
Cahill and Arteta have to be seen as incredible value for money along with other shrewd purchases of the Moyes era. I'd include Tim Howard who was a decent keeper for most of his time and served the club very well for many years.
For what he brought to the club and inspiration in many a dull game it has to be Cahill for me. Not the most technical player but had the bravery of Gray, tenacity of Parkinson and the love of the club that pretty much every player describes