Greece '04 is the only thing that comes close in my lifetime, I think, unless there's been some crazy upsets in the European domestic leagues I'm not aware of.
This is a substantially greater achievement than even that, however. A wonderful example to sport. Given the whole context, this has to be one of the greatest sporting achievements in modern history?
I hated that Greek side, mind. Bored me to tears. I've loved watching Leicester this season, full of guts and determination.