B&M are an interesting one. From a business perspective, I can't see them going anywhere soon. They build high quality, high capacity rides that consistently please both the parks and the general public. You rarely hear about a bad B&M.
That said, from an enthusiast perspective, I'm not sure I regard B&M as "one to watch" any more. They're capable of building good rides, but from what I hear and what I've experienced, they rarely seem to build anything really great these days. They're more than capable of building something really impressive, so hopefully they will pull something incredible out of the bag sooner or later, but right now the game is theirs to lose.
Intamin are a funny one. Their rides can vary from "mind blowingly awesome" to "ouch" to "meh". The Intamin gems really are amazing though, so I'd never count them out.
Most other steel coaster manufacturers seem to be let down either in the comfort, reliability or thrill stakes. I've yet to ride a really good GCI woodie, and don't think I've ridden a Gravity Group woodie yet, but if their latest creations are as great as they look, they're probably the ones to watch, at least in the immediate future. Now if one of them can build something as awesome as Boulder Dash in the UK in the next few years, that would be much appreciated. Ktnxbye.