I am not going to lie, it was a little hard to pick a "worst" of each of these companies because to me, for the most part they all make pretty solid attractions.
B&M: Kraken at SeaWorld Orlando.
Don't get me wrong, Kraken is a decent ride, it just was nothing special. It was not a very memorable coaster, and half the time I forget that it is even at the park.
Intamin: Fahrenheit at Hersheypark.
Again, Fahrenheit is by no means a "bad" roller coaster, but of all the Intamin roller coasters I have been on this one was the least memorable. Also, I may have been spoiled by Maverick so when I finally got a chance to ride Fahrenheit I was highly disappointed... Probably because I was young, and dumb, and expecting a similar ride to Maverick, and... well now I realize that would be like trying to compare Apples, and Poodles.
GCI: Wildcat at Hersheypark.
Yet again, Wildcat is not a "bad" coaster it was just very easy to forget. I rode it once and had no desire to ride it again. Not because it was bad, but because it was just... meh.