Boston with Tatum, could certainly make a pretty compelling package for Giannis this summer. Not sure Boston would have the best package out there, but certainly would be a top contender to land him.
So under the scenario where Giannis says he won't re-sign and the Bucks decide to trade him and assuming that every team below is essentially told Giannis will re-sign, these are the most compelling (and still realistic) trades I can come up with.
Boston - Tatum, Smart, Theis, Williams (either one), Memphis, 20 Bos, 22 Bos for Giannis
Philly - Simmons, Richardson, Thybulle, Smith, 20 OKC, 22 PHI for Giannis, Hill, Ilyasova
Toronto - Siakam, Anunoby, Powell, 20 Tor, 22 Tor for Giannis, Hill
Denver - Murray, Harris, Porter, 20 HOU, 22 Den, 24 Den for Giannis, Hill
Portland - McCollum, Nurkic, Little, Simons, 20 Por, 22 Por, 24 Por for Giannis, Hill, Ilyasova
Those to me are the 5 best trades where the acquiring team isn't gutted and Milwaukee still does very well. Obviously other pieces and picks can be included in any such trade.