Going to try to answer without reading previous comments

1. I think the best card is the Jx with x not being a spade. BU is obviously cbetting all spade draws and he can have a ton but BB doesn't have as many spade draws in his range and if he did he'd be raising a ton of them on the flop because basically all of them are combo draws including overs + FD. This also contradicts him having an OESD but I think he can flat here on the flop knowing that the BU can 3bet him often.

2. I think the best card is a K of not spades. At first I kept wanting to go with Ks but that kind of lowers the amount of Ks hands we have and we can have KsXs as well as like AsQs so it will help our range a ton instead of theirs.