Closest race I ever got credit for winning: tie. Checkmate.
The race was a hard arcade mode on the Test Course. I was in my Esperante GTR-1, completely stock save for the lowered ride height, max downforce, and no assistances. I was pit against a GT40 Race car, the R390, the GT-One, a Camaro LM, and the Vertigo. I started the race of odd enough, my lack of TCS kept me to the back, but I drafted the Vertigo and shot past the Camaro for 4th. What got me was that when I caught the main pack, this Putrid noen-green thing shot past at over 210 mph: the Camaro had stayed in my draft and had caught the main pack as well. To make a long story short, 5 of us were still dicing off the last corner, I was in the left lane being pushed by the GT-One. I couldn't see where the GT40 or R390 were, but I could hear both. I-we-blitzed across the line and my margin of victory was +0'00.000 over myself. Then the results screen showed my win tied the GT40. The replay is saved on my primary memory card.