BBEG. This makes perfect sense, they have lots of powers, lots of survivability, lots of minions to wade through along the way, usually some good lewtz and likely some Achilles' Heel that makes for fun games and great adventure stories. There isn't much room for discussion on "if" the BBEG is truly bad - they are the embodiment of evil made incarnate on some outer plane.
On the other hand is the Divine Being that is working counter to the party's goals. Now this isn't a true BBEG situation (at least not at first). Initially it is probably more of a speed bump for the party. However, knowing that a being is "good" but getting in the way is a great way to get a party wound up and invested in their characters and actions. Maybe the party is very lawful and the divine being functions in a very chaotic but ultimately good way - maybe the party is barely good and not really lawful, but the divine being is super good and super lawful - is the party really going to risk upsetting the "HIGHER POWERS OF GOOD" just to get a speed bump out of the way? Unlikely.
spider-web style of gaming and while it doesn't mesh perfectly with a sandbox environment, they can be very complimentary. So when the plan of the game begins to take shape, remember to include variables that can be adapted to the party's decisions. This is where the Good-BBEG (GBBEG), they don't start out as the BBEG, they may not finish as the BBEG or even ever actually be the BBEG. What they become is the *problem* that the party always has to consider when they come to a major decision.
They (GBBEG) cannot be used to often or the party will (rightfully) consider it a big problem and figure a way to neutralize it OR begin to feel like the divine being is a GM pet to be used whenever the GM wants to foil the party's plans. The GBBEG also cannot get involved in everything the party is doing or actually help the party in any meaningful way. The GBBEG has to be a BBEG in every way to the party, but not to anybody else. If the party saves the town from a horde of orcs, the GBBEG gets the credit, even if all they did was herd the peasants into a building to protect them (thus risking them all if the building caught on fire); if the party finds a big treasure haul, the GBBEG shows up to claim it as stolen goods from someone higher up; etc.
What happens eventually is that the party has to find a way to either extricate themselves from involvement with the GBBEG or find something better for the GBBEG to do - usually dealing with a true BBEG. The beauty of this is that then the GM gets to bring the GBBEG back as a new BBEG (corrupted from defeating a true BBEG) or as a more powerful GBBEG as the party advances. If the party decided to remove the GBBEG, well then they have to deal with the consequences of pissing off whoever the GBBEG was working for - the GBGBBEG (great-big good-big bad evil guy).