Pick the dearest one that will be the one with the least users and that is what makes mobile braodband usuable.
O2 is fairly good
3 is muck as is meteor
Using Vodafone myself at the minute its ok now at 2am during the day it is terrible.
But you take a chance with any of them because none of them can control how many users will be connected to a particular mast and all sharing the same limited bandwidth.