It's uncommon in Japanese to use the 2nd person "you," it's much more common to use the listener's name... although I guess you could "kimi" if talking to a pet, I'd still probably use the animal's name+chan, even when directly addressing them. Also, "ii" is more commonly spoken than "yoi," and people commonly say "ii ko" to say "good dog," although "ko" just means child. lastly there's not much need to be so polite with "desu," you can use the informal forms...
So I'd probably say something like:
fluffy-chan wa saikou no inu da ne! / fluffy is the best dog
fluffy-chan wa saikou ni ii ko/inu da ne / fluffy is the best child/dog (emphasis on being "good")
fluffy-chan wa hokano inu yori ii ko da ne / fluffy is a better "kid" than other dogs