With a dog whose friendly with other pets, which pet is best?

I have experience with rabbits and they are very messy and very scared of my dog even though my dog is very gentle with him. Ferrets are also very messy and will nip at your dog for they can be very aggressive towards animals other than another ferret. If I were you I would get a guinea pig or a rat because many of my friends Have dogs and guinea pigs that are best friends. Ratsare good with other animals though and are easily trainable. Good luck!!