My biggest win was being able to read a thread with a title in mixed capital letters (always hated this, but what can I do about it?). My biggest real win was having a tuned Suzuki GSX-R/4 create a complete upset over JGTC race cars. About a not really close victory had to be several times my B-Spec counterpart just edged out the 2nd Place driver.
If you want to go a bit further, my biggest defeat was the GT World Championship race at Hong Kong, the first time I've EVER been lapped in a GT game. Smallest defeat would be me trying to pull out an upset in the TVR One-Make race, and when I thought I won the race by a close margin, something rare happened- I lose by a small margin.
So, my biggest win, plus a little extra. Enjoy, my friend.