Best friends are the ultimate prank victims. Not only are their reactions always HILARIOUS, but out of BFF contract, they're obligated to still like you after it's done.
One time I sent my sister a recent picture of my best friend's high school ex-boyfriend and had her pretend it was the guy she was going on a Tinder date with. The next time we were all hanging out together, my sister started talking about the amazing guy she met on Tinder and showed us the picture of him on her phone.
My BFF suddenly got really nervous and said something along the lines of "...don't freak out, but that's my high school boyfriend." and then went into this speech about how she promises she's totally 'over it' and how she'd be super supportive if they ended up dating. ......And then we confessed our prank.
SHE WAS SO MAD. But also laughing SUPER hard. Her heart seriously stopped the moment she saw the picture of him on my sister's phone. She totally hated me for a good few minutes, but even then, she was able to laugh at herself.