The way that Simmons moves on the court and sees the floor, I feel he is the most unique of all of these. Yes, he has no touch right now. He may find it, he may not, but even if he doesn't, he is a 6'-10" Rondo who is only 21 years old.
Embiid is a classic big and could become dominant but injuries scare you with him. If I could pick all things equal, I would pick Simmons over Embiid.
As for as our guys (Brown and Tatum), love them both. Would not consider trading either unless we were talking Anthony Davis or someone like that. Although I feel Simmons has the tools to be the best of all, he also has more uncertainty. Brown and Tatum have a higher floor but perhaps not as high a ceiling.
In terms of the overall teams, I like that the Celtics are where they are without tanking. Short term, the Celtics are better because we know we are getting Kyrie and Hayward next season. Philly could add a big name and change the equation but who knows.