First post, here we go!
With our current roster (without including the potential Hayward/George stuff), I think the strongest starting five would be:
IT
Bradley
Crowder
Tatum
Horford
I think Tatum can play immediately with some success. Considering we don't have any terrific PFs to compete with him, I figure it won't hurt to try him out and see how he does against NBA bigs. The primary concern is how he does with rebounding, but I can't imagine he'll do any worse than our other options.
Second unit is some combination of Smart, Brown, Semi, KO, and Zizic. Possibly throw in Rozier here and there.