I've always been deeply impressed by Final Fantasy 14's solo only challenge runner RathGames—who has been working through the MSQ on his lonesome. Or, as much as is possible given the circumstances. See, FF14 is an MMO, it's not meant to be done on your own, there are explicitly mechanics, encounters, damage checks that are designed for a full party of four to eight people.
Furthermore, RathGames refuses equipment from quest rewards, making most of his gear himself—the one time he's had to renege on his promise was within the Crystal Tower, in which he assembled a group of other solo-only challenge runners because the game quite literally wouldn't let him inside otherwise.
