Ohhhh, this is something of a specialist subject given the fact that I've written a gabillion speeches for weddings in my other biz. Some classics:

• Advising that we shouldn't include one of the most graphic stories I've ever been told which basically consisted of the bride sat drunk in nothing but a bra and a convoluted punchline that revolved around (what I hope was a fictitious) gangbang. As told to me by THE FATHER OF THE BRIDE who wanted to include it in his speech.
• So, so, so, so many best men who say "But the lads are all expecting me to rip him to pieces" and then proceed to argue that we should include endless recycled gags including the "going down on the queen…" gag.
• Best men who when you say, "There will be grannies and children listening, are you comfortable saying this to them." start to get a bit shaky.
• Mentioning exes. Brides are basically buttons that cry when you push them – don't fucking press them because you don't want to be the guy who made her cry on her biggest ever day. Your mate really won't thank you either. 

It all gets a bit tiring after a while. The best speeches are the ones that include humour (not gags) but also talk honestly and nicely about the relationship.

Being asked to give a best man speech is a lot like being asked to go down on the Queen. It's a great honour but no one really wants to do it.

It's not even that funny.

Exes – my opinion is that it's just better to completely avoid. The point is that you just don't know how sensitive the bride's cry trigger is – make like Oprah's mum and give it a wide berth.