What do you think has been the "best" purchase of a startup?

Was discussing with a friend. What do you think has been the "best" purchase of a startup so far.

Main criteria for "best" being that it was a young startup, not like the YouTube purchase or someone that was already huge and successful, but also "best" meaning it involved a huge sum of money for the startup, enough to let them retire or at least consider themselves successful.

If anything, it's a fun thing to think about and research. I just lost an hour Wiki'ing various purchases over the last decade or so.