Can't decide on an order, but the top 5 for me are: Easton, Maclean, Mannus, May and O'Halloran.
I put O'Halloran in purely on his quality-per-minute. For the lack of pitch time, when he has been on, he's been fantastic. As with Easton, he's had to fight his way into the squad and it's not been an easy journey.