It's better to have a pair, because rabbits are very very social animals. A lone bunny can get bored, anxious, and very very destructive. The trade off is that the pair may not be as cuddly and attentive to their owner as a lone bunny, but they will be immeasurably happier.

Cats are predators, and rabbits are prey animals. You can have cats and rabbits together but it's not recommended - it takes less than a second for Kitty to go from "this is a friend" to "this is dinner", or for BunBun to literally kick the guts out of its former pal. Same thing with dogs; bunnies can and will end up on the wrong end of the prey instinct and can either get destroyed or end up dead of shock/fright. And people are not fast enough to keep these things from happening in 99% of cases.

Side note: some people keep rabbits and guinea pigs together. This is a terrible idea because even though their diets and habits are similar, guinea pigs carry bacteria in their guts that can absolutely fuck up a bunny.