I like none of our options. Gun to my head, I'd vote for Semi.
Horford was doing a decent job on Giannis. Obviously, he couldn't stop him 1-on-1, but no one can stop Giannis 1-on-1.
Here's a question: Who do you guys think is the best Giannis "stopper" in the league?
My default go-to would probably be Kawhi, mainly because he was the key player to stopping Giannis in the playoffs, which is IMO the most impressive (and useful) place to stop him. But I think using the term "stop" for someone this dominant is never really right. You either slow them down as much as possible, or try and shut down their #2. We have a seeming complete inability to slow down Middleton, so our best bet might be trying to slow him down.
Our best bet might be finding someone in the draft, given Giannis will (in all likelihood) be around for a while. CFAN38 mentioned Pons, I think Paul Reed could give it a red hot go. 6'9", 7'2" wingspan, can play the passing lanes, block perimeter jumpshots and is a great rim-protector. His defensive impact reminds me of a lesser Nerlens Noel. I think his career will play out very similar to Jerami Grant (overlooked in the draft, goes on to be a steal - Grant has been better than ~50% of the lottery picks from that year), but with better rebounding, and in all likelihood better defensive numbers. He'll probably never become a 40% 3 point shooter like Grant though.