Tim Howard from MLS to Manchester United at their peak is probably still the biggest signing. Among field players, Jonathon Spector to Man U in their prime, Erik Palmer-Brown to City a few years ago was huge. Chelsea and Juve are about the same but admittedly one stock is up and the other is down. Brad Friedel to Liverpool after three previous clubs couldn’t get him a permit was a big deal. Onyewu to AC Milan was a significant Serie A signing, if for no other reason than as soon as he got there he threatened to sue Anderlecht and Belgium soccer for failing to stop on pitch racism. As soon as that was calmed down, he played in the Champions League, punched Zlatan and then got hurt just before the World Cup. That was a blessing though. It opened the door for DeMerit. Him signing with Watford without any pro experience was a miracle btw. Highly recommend the biography of his life.
I’ll see if I can think of more.
It’s cool he signed for Juve, and exciting, but there are three USA youngsters signed to Bayern playing reserves and youth. Each of those is arguably more significant and might see time before McKinnie, especially with his injury history.