Of those 3, Simmons is the clear choice given his unique skill set.
I'm not so sure that Markkanen won't end up better than them all though.
Then you have Ingram, who has been an absolute monster so far in New Orleans.
So right now I think based on all I know of those 5, I'd have Simmons, Ingram, Tatum, Markkanen, Siakam. Wouldn't be surprised if in a couple of years the order was all mixed up though.