I don't recal tamas having a fav toy, but all girl tamas like the teddy bear and all of the boy tamas like the action figure. They like these toys from baby to adult.
Actually, I don't believe that adults play with the teddy bear/action figure. The only thing I really know is that they don't sleep with 'em like children do. Give an adult Tamagotchi any item, and it'll most likely entertain itself. The only downside is that you can't make the happiness meter go up by making them play with toys.