My cousin told me I could have one of his accounts so I was just wondering which class would be best for boosting a new character. He has a 70 mage, paladin, and warlock.
DO NOT DO IT.

You MUST have all accounts in YOUR name, otherwise there's a very strong likelihood that you will get banned. There are threads all over the forum about this, and it's very important.

Your only hope is if your cousin happens to share your surname, which would still be against the ToS but less likely to be found out.

All of the classes named are viable for boosting, and have been discussed at length in various parts of these forums.