the or not?

Yes, you need "the" here.

Anonymousthe or not?

the

1. dog is not a proper noun.
2. dog is not an abstract noun.
3. dog is countable.
4. dog is singular.
5. dog does not represent a role.

Once all of those apply, you must have a determiner; usually it's a(n) or the.

CJ

To add to my previous answer: this would be appropriate to say if you have more than one pet, including a dog, and the dog is your favorite among them.
If you mean you think that dogs are the best kind of pet, you could say, for example:
Dogs are my favorite pets.
My favorite pets are dogs.
Dogs make the best pets.
Dogs are the best pets you can have.

CalifJimOnce all of those apply, you must have a determiner

I agree. But sometimes I find counter examples, which cannot be typographical errors, given the frequency I see them. For example, William Weaver would say

-His profession is gravedigger.

Do you agree with me that the verb be has a privilege

OttoJFor example, William Weaver would say-His profession is gravedigger.

See 5. "Role".

Also, many fixed expressions that start with prepositions have no articles.

CJ