From SE23 then very early or late and the South Circular is probably the best. But on a Friday afternoon I don't think there's such a thing as a "best" route. All routes out of London are really busy.

On a Thursday afternoon if motorway traffic is flowing ok then you could go out the Bromley route to M25 and then around to M3. The only problem with any motorway route is in the event of roadworks or accident slowing you down to a crawl, then you're completely stuffed until the next exit. So check up on traffic news first etc.

If time is not important (ie, a pleasant drive) and you are happy to avoid the motorway, then one of the nicest routes is out through Biggin Hill and down to Westerham where you can pick up the A25.

This runs parallel with the M25 and will take you through Godstone, Redhill, Reigate, Dorking, Guildford and then you go over the Hogs Back and down to Farnham. From Farnham you cut across to the M3. But then you're only on the M3 for one or two Junctions before you turn off again for either the A30 or the A303 for the South West. It's a very relaxed drive and great if you don't like motorways.

It all depends on whether your "best" is the quickest or nicest route.