To start, I posted this once awhile ago, but Gravity Falls is probably my all time favorite cartoon, and I love their cover, so here it is again.
As you can tell, they always dress up like the characters from whatever piece of media they are covering, and when a piece of media has a prominent female character, they will sometimes bring in a guest singer, like this cover of John Williams' Harry Potter theme, where they brought in my favorite YouTube baker who also sings, Rosanna Pansino. :)
And from one John Williams theme to another, (I swear I didn't plan this!) my all time favorite Star Wars theme, Duel of the Fates!
One of my all time favorite shows (that only premiered last year) is Stranger Things, and I love this cover of the theme music. :)
And last but not least, from one Netflix show to another, my latest absolute favorite show, (And I'm not being hyperbolous, I love this show SO MUCH) the theme to Netflix's A Series of Unfortunate Events.