Willie has probably been the most consistently good forward on the team. Despite his lapses, Matthews is the best overall. He's just so good at the most important skill (scoring goals). His production there elevates him. After that, it's between Willie and Kerfoot for me, but the latter has taken a ton of penalties. Mikhayev and Kapanen have also played strong. Mitch has looked better the last two, but his defensive play has been just horrific all season. Apart from shredding a few bottom-dwellers, he's looked bad. With that said, he'll rebound, and so will Johnny T.
There's no question that the main problems with this team right now are JT, Marner and Barrie. All 3 are playing significantly below what is to be expected from them. Hopefully they straighten up soon.