Tony Kanaan finally winning the Indianapolis 500 in 2013, after countless, fruitless attempts.
Will Power shedding his "can't do well on an oval" and "can't finish off the season" images with an impressive victory at the 2013 500 mile season finale in Fontana, holding off the previous two 500 mile race winners (Ed Carpenter at Fontana in 2012 and Kanaan at Indy in 2013). While Power was well out of championship contention by this point, winning three of the last five races including Fontana helped salvage enough to get him back to 4th overall in the season.
The box score says Ryan Hunter-Reay led the last three laps of the 2014 Indianapolis 500, but that doesn't mean he didn't work for it. Here's the run to the checkers.
Nigel Mansell winning the 1993 Championship, he did this despite a back injury he sustained at Phoenix which troubled him throughout the season. He won on his debut in Australia (a country he usually struggled at in F1). He'd never raced on an oval before; and won 4 of them! he was going up against series veterans like Andretti, Fitipaldi, Tracey, Al Unser Jr, etc. He'd never even sat in an Indycar prior to 1993 and still came out on top. He remains the only man to hold the Formula 1 and Indycar titles at the same time.