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.