Think everyone knows its Zabaleta, because he isnt the most gifted or stellar player we have, but has excelled and given his all all season.
All the 'top players' have been complacent this season, coasted along at times, whilst Zabba has always shown he desire and commitment to the club, team and fans. That's what makes him the best player of the season, not just someone who's having a good game once in a while.