I based the low-tiers off of the bottom 10 characters on the current tier list, so everyone below Young Link. I chose below Young Link since he actually has top players who have used him or still use him like Axe & Armada. The bottom 10 chars: DK, Link, G&W, Roy, Mewtwo, Zelda, Ness, Pichu, Bowser, Kirby.
I couldn't find upset sets for Kirby, Pichu, Ness or Roy FeelsBadMan. I wanted the upsets to at least be on MIOM-ranked players, Triple R has defeated AbsentPage on stream but it was when Triple R was actually ranked above him in Minnesota. Mofo beat Gravy but it was before Gravy was as well-known as he is now, and in 2013 before he was on the MIOM rank. Even D1 and Prog the commentators didn't know who he was then, but called Mofo the best Ness in the world. I could not find footage of NEO using Roy and winning in sets that could be considered "upsets." As for Pichu, someone save that poor soul.
Taj vs Colbol: This is a very impressive win for Taj because he had been relatively inactive since 2013, but Taj showed up at TBH6 placing 33rd using only Mewtwo. Beating Colbol was very impressive, especially considering he was ranked 25th in that year. You can never count Taj out though since he has been known to surprise everyone, I really hope he enters a singles tournament this year regardless of what character he picks.
Sixx vs Amsah: Amsah is simply a legend of the game, so when Sixx managed to 3-0 him, a player who many hadn't heard of since he rarely enters singles events, it was shocking. Sixx also beat HMW at Heir 4 but I decided to choose win on Amsah over HMW (another 3-0) since I find it more impressive.
Narcissa vs MacD: While MacD wasn't as well-known back in 2010, (the tournament was POE3), Narcissa defeating him was a great achievement for Zelda. MacD went on to place 25th at Apex 2010 a few months later and would become considered a top ~25 player in 2011. It would only be until 2018 that another top Peach would be upset by a Zelda with Plup defeating Armada at GENESIS 5, proving that Zelda beats Peach 60-40.
Iori vs Overtriforce: Iori's ability to play the whole cast at a high level is sure to bring a tear to the eye. He placed 3rd at POE3 which I mentioned above, and in this set he decides to use Donkey Kong vs Overtriforce and manages to win pretty convincingly. Iori has yet to be MIOM ranked mostly due to just not traveling but is certainly a hidden-boss and a player you shouldn't sleep on.
Zoma vs Cactuar: Japan's best Mewtwo who does not get enough recognition, Zoma, defeats Cactuar (MIOM #64 that year) while on his trip in Japan. Zoma went on to win the tournament, defeating Shippu with Fox and Mewtwo in grands. The last time Zoma went to the U.S. was in 2016 for Smash Con where he placed a very respectable 33rd, I hope to see him come to the U.S. to compete again soon.
Qerb vs The Moon: This is not the only time Qerb has beaten The Moon on stream, the other time being at Break Smash #66. I chose this set in particular though since it was his first time beating La Luna afaik, and also because he finished the set with an iconic 9. I didn't want to include La Luna again in this video but he also lost to SAUS' Link at GOML 2015 if anyone is interested in watching that.
DJ Nintendo vs Ryan Ford (formerly Unknown522): DJ Nintendo goes down a game and like a madman, he brings out Bowser and wins very convincingly. Ryan Ford was ranked 15th that year, just one spot below Leffen, easily making this Bowser's best win.
Bum vs ChuDat: Should be noted Bum also beat Isai this tournament, but I didn't want to include that in here because I was afraid of how many comments would be saying "but Isai sandbagged." Anyway, Bum placed 4th at this national using DK, making this DK's best performance at a tournament. In this set, ChuDat decides to go Pikachu and YL... Ice Climbers are not great on Rainbow Cruise which would be why he avoided going ICs there, and YL does well vs DK. As for why he went Pikachu, well... they both have Chu in their name.
Cereal Rabbit vs Armada footage has been lost except for the last 9 seconds. Sad.
Armada was drunk and used Pichu/Sheik for those who don't know about the legendary set.