I recently heard back from a client I was writing for that said that a particular line in the best man speech, and I quote 'wasn't harsh enough for his humour'. This is symptomatic of the approach that so many best men take, and that's thinking that the speech should be some form of public humiliation. So many best men think that they have to tear the groom apart, and that involves the kind of humour and stories that they'd share in the pub. What they're really thinking is that they'd like to look ruthless and cool in front of the bridesmaids, and reap the rewards afterwards. Well, what it boils down to is: just how much of an idiot do you want to look?
You're there to celebrate the groom is a really funny way, and only genuinely funny will do. It's quite possible to make everyone really laugh, but recounting explicit, crass stories isn't it, and peppering the speech with profanities won't do the trick either. Just think: if you wouldn't read it out to your granny, then don't have it in the speech. If you're thinking 'well, that doesn't leave me with anything funny', then you're just not thinking about your content in the right way. You have to be a lot more creative when trying to make everyone laugh, but it's worth the effort. The best man speech is never about the best man, so stop thinking about what works for you, and start thinking about what would work for the happy couple. I have countless stories about when the best man thought it would be great to destroy the groom, and it never ends well.
So, instead of thinking about how to be harsh, brash and macho, instead think about how to be genuinely funny - there's a big difference. With the former you'll probably make 6 people laugh, and the rest of the guests think you're a knob, and with the latter, everyone will spend the rest of the evening saying how great you. Not much of a choice to make really is it?