Particularly... Hard to choose, but here are a few. I'm planning to use this thread as a very slow blog to dump my bookmarks into, check it once a month or so.

https://www.youtube.com/watch?v=M58KwXYaDII&list=PL5D11CCA56D3E6E69&index=2
A Super Mario Thing, or ASMT for short.
by: raocow

This is a Super Mario World hack, made specifically for raocow by his fans. Probably his best series ever. Full of inside jokes and ancient memes.

https://www.nicovideo.jp/watch/sm6837901
VIPマリオ3 letsplay. In japanese, but somewhat entertaining nonetheless. Also a mario hack. It's a very challenging hack, at times sadistic even. But not impossible. Kinda. Maybe. See it for yourself.

Pretty sure raocow watched either this series, or something like this, because he bragged about "introducing VIP to the west" once, because he made a video saying that playing VIP blind(without prior knowledge of the game, and/or translation) is a bad idea, and just his editing style is similar.

https://www.youtube.com/watch?v=KUqg8XtAHnc&list=PLQDWoXFQ-YLpeEFkzeDZF1MmNQS7BDoI4
Dark Souls letsplay by Markus, aka EpicNameBro. A winner in the "most effort" category, explains every little plot detail in the game.

https://www.youtube.com/watch?v=Pez0in248jg&list=PLRQGRBgN_Enrz7rF2Ozm0_qQ_kbz0_kdc
Chu♥lip letsplay by GameGrumps. A winner in the "funniest" category.