Best Celtic product?

Since 2000, who has been the best player to have played for Celtic and then went on to have great success after being transferred out?

Matt O’Riley and Kyogo seem to have not been faring too well so far, granted it’s been a short period of time, but do we generally send off top quality or players who can’t hack it? And who of all has been the best player that was purchased from us? Would imagine it’s Virgil van Dijk?

Edit: to clarify I mean the player who had the best career after leaving Celtic.