ChessaQ, are you perhaps thinking of pitbulls rather than bull terriers?I would say a bull terrier would be more difficult. They have a tendency towards aggression, and are very strong willed. I don't think I would recommend them as a first dog.
Rotties can actually be pretty easy dogs, but some are harder and can be more difficult to train. And of course, they're incredibly strong. Of the three, I think I'd pick a beagle for a first time dog owner. No dog is easy, but they're probably the best to start with.