If you ask MotoGP fans to mention what their favorite or most notorious race is, quite often the answer is Laguna Seca 2008, since everybody remembers the amazing overtake over Casey Stoner at the most famous set of corners of perhaps all the MotoGP circuits, the Corkscrew; or maybe Catalunya 2009, which will always be remembered for being an epic battle with Jorge Lorenzo that left us with numerous overtakes, and a grand finale for the ages, with Rossi getting ahead of Lorenzo using an "impossible" move; a race for the ages and Rossi's favorite last overtake; or perhaps Welkom, South Africa, 2004, which was so much more than a victory over Max Biaggi, this race was Valentino's first win with Yamaha after moving from Honda, which was deemed a bad decision since the Yamaha was an inferior motorcycle and Honda had the best bike in the paddock, but Rossi, who was already a 5-time world champion, wanted to prove to all his critics that he was winning races and championships not because he had the best bike but because he was the most talented rider in the MotoGP grid, the South African race is Rossi's favorite race/win and he has refered to it as "my masterpiece".
While those might be considered by many some of his most notorious victories, there are so many others that are worth mentioning such as the victory over Andrea Dovizioso in Qatar 2015, where Rossi, with an inferior Yamaha, managed to beat a very talented rider with a superior motorcycle in a strategic race; or Germany 2006, where he won the race from the 10th position in the grid (which might get an article of its own, since the race is available for free on the MotoGP YouTube channel). There is one race that is stuck in my memory: Australia 2003.
By the time the championship got to what was known as the "Pacific races" (Japan, Malaysia & Australia), Valentino Rossi had so many points over his rivals, that he had a chance to wrap up the championship, which he did in the Malaysian grand prix. For the Australian GP, Rossi, being such a MotoGP fan himself, was wearing a small badge with the number 7 on his leathers, in honor of the late Australian rider Barry Sheen. Leaving from the pole position, Valentino, with the title already in his pocket since the previous race, had already announced that he wouldn't be riding for Honda the following year, the Italian rider had nothing to prove, but he was the favorite to win the Australian race. The scenic Phillip Island circuit with its stunning views over the ocean and its fast-flowing corners, is one of the most loved circuits in the world championship calendar by many riders, including the Italian world champion.
*If you plan to watch the race, I suggest you stop reading now as the following lines would reveal the resolution of the race.
The Italian rider didn't have the greatest starts from the pole position and he was riding in the 6th position for the first couple of laps, but he quickly started gaining position, passing other riders. On lap 3, local rider Troy Bayliss suffered a spectacular fall that left him still on the grass next to the track, During the following lap, Valentino Rossi continued his climb to the top, overtaking Marco Melandri, to reach the 2nd position. A few corners later, Rossi overtook Nicky Hayden to lead the race. As Valentino got to the 1st position, he started to open up a gap with the group behind him. With Rossi so far ahead of his rival, the interest of the race was on the battle for 2nd. With 15 laps to go Valentino Rossi received a 10-second penalty, for having overtaken Marco Melandri while the yellow flag was being waved (since Troy Bayliss was still being looked after by the medical staff on the grass area right next to the track). As Valentino crossed the finishing line after that lap he immediately saw this information on his board, and having a 3.5-second gap with the following group, he needed to increase it to 10 seconds if he wanted to win the race, which seemed like a mammoth task. Effectively, Valentino, though running alone at the head of the race, was in 8th position. Shortly after this, Marco Melandri fell, leaving Loris Capirossi in a solitary 1st position (though, on the circuit, he has physically 2nd after Rossi). With 7 laps to go, Loris Capirossi is told by his board, that he is in the 1st position and that he has an advantage over the 2nd rider. Rossi was chipping time away from the gap that separated him from Capirossi, and with 4 laps to go, he managed to have a longer-than-10-second lead, hence leading the race once again. He finished the race physically more than 15 seconds ahead of his compatriot Capirossi, effectively over 5 seconds after the penalty. It was an absolute display of sheer talent and race management. Valentino did his victory lap waving a flag with a big number 7 in honor of the late Barry Sheene. It was truly a race for the ages.