Stirring emotion and rousing sentiment
From the outset, you should make it abundantly clear to the guests that you intend for this speech to be a genuine and heartfelt one, not a bag of laughs. You might want to start with a sentimental icebreaker (SEE EXAMPLES), then thank everyone for being present today (SEE EXAMPLES). You can then compliment anyone else who has already made a speech, or intends to (SEE EXAMPLES).
In order to sound extra-intelligent and meaningful, it's not a bad idea to use a famous quote about marriage or love (SEE EXAMPLES), and then to describe some of the emotions you had while you were watching the wedding today (SEE EXAMPLES).
Thank the groom for the great honour of asking you to be his best man and let him know how much it means to you (SEE EXAMPLES). Tell people how delighted you are for your friend (SEE EXAMPLES), how he's been someone to rely on and cherish over the years (SEE EXAMPLES) and even how he's inspired you in your own life (SEE EXAMPLES). At this point, you could pull out a nice quote about friendship (SEE EXAMPLES).
Next, it's time to talk about the groom's new wife, and how you always knew she was the one for him (SEE EXAMPLES). Pay the bride a warm compliment or two (SEE EXAMPLES) and describe how as a couple they just seem to be made for each other (SEE EXAMPLES). Suggest what kind of married bliss awaits them (SEE EXAMPLES).
Thank anyone else who has had a hand in today's festivities (SEE EXAMPLES) and tell people how you have been looking forward to this big day and are now looking forward to the remainder of the festivities - especially that this speech is now almost over (SEE EXAMPLES)!
Humbly suggest that people might want to get on with their evening rather than listen to you go on for much longer, and say you will now bring your speech to an end (SEE EXAMPLES). But, just before you do, reassert your best wishes to the bride and groom (SEE EXAMPLES), then implore everyone else to do the same (SEE EXAMPLES).