In times gone by, the best man role was usually handed over to a brother, or a good friend, and whilst that's still largely the case, things are changing. It's not unusual for it to now be the responsibility of anywhere up to 4 friends, 3 brother, or indeed a combination of the two. Lots of guys are choosing a close female friend or sister to represent them, which is great - the only problem is how to refer to them. Best woman, seems a little too much like an award title, whilst they're clearly not a man. This is something which is, as yet, unresolved and will probably stay that way until someone has a brainwave.
Increasingly, however, fathers of the groom are being asked to be best man. Many sons are lucky enough to count their dads as one of their best friends, and have super close relationships with them; there's also the added bonus of the fact that very unlikely to know anything about the time you visited the Ping Pong Strip Club. So, is there any difference between a father writing a best man speech, and say, a best friend? Well, it does depend on what your relationship is like, but the majority of speeches I write for this scenario do have a different feel to them.
Whilst I still like to weave everything into one big story, that does tend to be a lot more tongue in cheek, and feels less appropriate for a father making that speech. By virtue of the bond that ties you together, it will always be a slightly more emotional and meaningful look at who the groom is, and is more like a father of the bride speech. So, instead where I'd use a little fantasy and creativity to stitch things together, the father best man speech is more of a plotted look at key points in his son's life, obviously with as much humour as you can cram in there. Making it an edgy romp through the more sordid parts of the groom's life is never a good idea whatever relationship the best man and groom has, however, the father saying anything remotely edgy about his son can feel incredibly awkward and forced.
The bottom line is: every best man speech should be an entertaining look at who the groom is. but when the father is making that speech, there's nothing wrong with it being more of an emotional and celebratory look at what made the groom the man he is.