I know it's hindsight, but the following FAs are guys I wish we had signed on the cheap. At least one of them:
Boogie Cousins--He would have fit so well into our system IMO. He has really nice outside stroke, is stronger lower frame who doesn't get pushed around. Plays hard. Tons of experience but not too old. Expressed interested in signing with us before he inked with GSW a while back.
Josh Jackson--He was my draft binkie back in 2017. I valued him above Tatum, which was a huge mistake. But character/maturity issues really hindered his development. He seems to have grown up a bit. His last outing for the Pistons was impressive, on both sides of the ball. He is a 6'8" swing forward who can defend, is very athletic, his shot is finally improving. He would really fill a hole for us right now. Could have signed him for peanuts.
Harry Giles--His last game was soft, but overall I think he would have given us more length, athleticism, etc. Still very young.
I'm sure there are others. Who is on your list?