Probably not the *most* hilarious but something I've enjoyed for a while... This guild on my server, they're like really casual and social but they've been around for a while and their recruitment messages on Trade are always like "All we do in raids is wipe and noob out! Come join us and let's be noobs and wipe together!" plus some messages like "Wanna know how to make a cake fast? We know! Throw a bag a flour and baking powder on a Fire Mage and voyla!"
Back in Classic, I had just finished submitting a college app and decided to hop on WoW. I meant to copy/paste something else, but accidentally copy/pasted my college essay.
When 2 people argue in the trade chat especially on server who has trolls on the trade chat and your like... "Oh this is gonna be good" and people start flaming the guy.