From Asia I usually use the "jaist" one (which is from Japan and the fastest one here afaik) as that one from Taiwan seems to blocked here.

EDIT: to get the user's country, just check the LANG environment variable for values like "en_US", "zh_CN", etc. Or you can also check for timezones.

I tend to use the .ac.uk one, it is really nice and fast, but that is because it is only 70 miles down the road

I find switch (swiss) is a lot faster and more reliable than mesh (german) for me (it is possible that the speed comes mostly from proximity (I'm closer to switzerland than to larger German cities except München (Munich)...

I also usually use Japan, sometimes Taiwan and sometimes also Dusseldorf, Germany. Doesn't really matter for me if it's 5KB slower or faster. Most of the time I don't notice a real difference in speed.

I tend to choose randomly. Haven't really noticed the big differences in speed, maybe because it's hard to notice with my kind of connection (hybrid between EDGE and satellite).

As for the closeness of a mirror I don't think it matters for me anymore because download goes over satelite and routes through Luxembourg (I think) so whatever mirror I choose it will always have to contact the satelite first, so I don't really gain anything by choosing a nearby mirror.

EDIT: Actually I might be wrong. Maybe I should just choose something close to Luxembourg if that's the routing point.

I use the Irish mirror (called something like HAENet).