It's a toss up between Mack and B&M for favourite coaster designer. Whilst B&M's can be formulaic, it doesn't necessarily mean they are bad rides. I was very surprised by just how good rides like Scream, Katun and Medusa are and when B&M go for it (Tatsu, Nemesis, Riddlers Revenge), they really go for it. Mack are up there because of the awesomeoness of Blue Fire (And soon Helix) and the sheer amount of rides they design and build.
Intamin rides are okay but for every Juvelen there's a Tornado or Furius Baco.
Flat ride manufacturer, it has to be Huss who designed so many staple flat rides of my childhood. Pirate Boats, Enterprises, Top-spins and Breakdances are just