This is a really tough choice, pitting current distance from NHL readiness, and high floor vs. further away but higher ceiling. It's really a paint the ceiling with peanut butter and toss up noodles and see which one sticks.
I went with Mikkola, as I think he has the tools to eventually be a very solid 3rd pairing NHL defenceman, and, with a lot of hard work and dedication, he might eventually top out as an adequate #4. Despite Lindbohm being a lot closer, already show he can play well (at times-though inconsistently so far) on The NHL level, he looks to top out as only a low-end regular 3rd pairing player. While Mikkola looks to become a strong #5, or even sneak into a regular #4 position. So, he gets the nod.
His higher ceiling beats out the more NHL ready Sundqvist, who should be a good 4th Liner, but not too likely to be anything better. I don't see Blais being more than an AHL/NHL tweener, at best. Binnington is iffy enough just being a young goaltender. But he's been too inconsistent to hang one's hat upon.
Musil and Toropchenko could be right up there with Mikkola for different reasons.
Musil looks to have a high floor, having several talents in different areas of the game (good at defence, forecheck, cycling, doesn't shy from physical game, halfway decent shot, decent puck skills, decent hockey sense, and can play all forward positions). He looks to become an excellent 4th Line NHL centre, and with hard work and continued dedication, might top out as an adequate 3rd Line forward.
Toropchenko has very good offensive sense and adequate skills (decent puckhandling, decent passer, decent shot, good skater). He could be an offensive diamond in the rough, if he improves his puckhandling, passing and shooting to the next higher level. To me, he's got the highest ceiling potential of anyone left. That puts him up with Musil and a hair below Mikkola (only because evenly competent defencemen are worth more than their counterpart forwards).