in (one's) (own) interest
Beneficial to one. It would be in your interest to have your lawyer review the contract before you sign it.
in one's (own) (best) interest(s)
to one's advantage; as a benefit to oneself. It is not in your own interests to share your ideas with Jack. He will say that they are his. Jane thought it was in the best interest of her friend to tell his mother about his illness.