Im going to say Simmons.
Fultz is a very nice player who lools to have a Damian Lillard like ceiling, but Simmons' ceiling is so much higher. He's probably a bigger risk as well because of personality and his one glaring limitation (shooting), but in a "stars align" there are very few players ive seen in the past 30-40 years who have the type of potential Simmons has...and as a bonus he's a natural Pf, which is the one position we are most desperate for.
Fultz is a very talented all round player, but his size limits his versatility a little. He can play pg and SG, but that's really it - he's too small to play the SF spot (at 6'4" or so).
Simmons could play positions 1 through 5 and he can play then all relatively easily. Very few players in history that have been able to do that. You have Magic, LeBron, Odom - off the top of my head I can't think of any others, and even Odom is a squeeze because he'd only be able to play pg or C in emergency minutes...while Magic, LeBron or Simmons could start at either Pg or C for a full season if they had to.
Even other highly versatile guys like Penny, TMac, Paul George and Pippen couldn't dream of starting at Center for a full season.
A combination of size and talent like that just can't be tought. That's a God given gift, and if he could imperative his jumper and his attitide his potential is seriously limitless.