I'd actually put Kalyn at #1 considering we were the worst team ever when we pulled that off. Never harder to sign a good player. And it was something of a punt, no one had spent that much on an unproven teen before (we sort of set a new precedent there).
But those are the only two in the conversation for sure. With BK it's an elite player in his prime who was at a much worse club, and we were in a great cap position at the time due to a bit of a mini squad recycle (a lot of the great players in that team were on their first contract). It's the kind of signing good teams often make.
Our next best ever signing is probably Dane Gagai (the first time).