I have the same problem every spring in the NCAA hockey tournament bracket challenge here...
My greatest predictions (of any kind): loudly announcing to my broomball team that Lebron James would become a bigger NBA bust than Sam Bowie. I still hear about that one.
Also, my senior year roommate (moving back in the frat house in August 1991) returned from his internship in Seattle with a bootleg CD of some band I'd never heard of. After about 30 seconds of listening to it, I took it out of my CD player and put Motley Crue's "Decade of Decadence" back in. Then I went on to tell him that hair band metal was here to stay forever and that this "grunge" look of his and that crap CD "Nirvana" would be gone in a week.