However much you are prepared to take the Mickey, you absolutely have to say at least one genuinely nice and meaningful thing about the groom. Don't go over the top, though; you're not giving a eulogy.
There are plenty of ways to give a compliment without seeming mawkish. You could even tee the tribute up with a joke:
"I've known the groom for a number of years and of course there are plenty of things I could tell you about what he got up to in his younger days. However, at the bride's request I've been asked to keep things to a minimum... and only talk about his positive attributes."
You could highlight the groom's best quality and relate it to the occasion in some way:
If he has a reputation for being Smart/Intellignet …
"Of course, marriage for everybody is a major adjustment. For example… before today, as a smart single man, Paul had the answers to everything. Now, as a married man, he has the answers to nothing."
If he's Entrepreneurial …
"I've been a first-hand observer of Paul for a long time, being a close friend of his, and I've been drawn into many of his wild schemes as an unwilling ally. His ventures were always adventures. He was in his element when he'd fastened onto a new idea, no matter how harebrained the idea was. Although many of his early business initiatives failed he triumphed in the most important enterprise of all - his courtship of Linda."
If he's renowned for his Generosity …
"Paul, it's a real honour to have been chosen to be your best man today. I think it reflects the fact that we've had some unforgettable times together - and you truly are a best friend. I don't think I know of anyone more generous - generous in giving help, in advice, or in the pub. That last one may have to be reigned in a bit now you are married, but only from tomorrow."
If you're in touch with your own feelings in a modern and manly way, you could talk about what a great friend the groom is, how he inspires you, how he'll make a great husband, how you are honoured to know him and how privileged you feel to be his best man. Just don't cry, or you'll never live it down.
If you haven't done so already, compliment the bride. If you have, feel free to do it again. And if you still feel like ribbing him, there may be an opportunity to get a laugh at the groom's expense at the same time.
Express your approval of his choice of bride …
"I can't imagine a couple more ready, more right for this blessing. And I couldn't ever have imagined a bride more perfect for Paul - thank you, Linda, for bringing him and all of us such happiness."
Pay tribute to her efforts in organising the day …
"Linda took on another full time job recently: planning the wedding. And I must say that while the pay isn't good, the hours are long, and the work isn't glamorous, the returns will be everlasting."
Suggest how the bride's influence has improved the groom …
"The splendid day we are all enjoying has only come about because of the love between Paul and Linda and their decision to show their commitment to each other through marriage. I have to say that Linda has been a wonderful influence on Paul. Since the day that they met, he has been a happy little soul and I wish them both all the future happiness they deserve."
If you're the sentimental type, you might want to compliment the newlyweds on being a model couple or a great match. Personally, unless you're a particularly emotional individual, I'd save that for the round-up at the close of your speech.