Orange + T-mobile, because I work for them and have access to the trial joint network

. I have used all 5 at different times for testing and there isn't really a best or worst - they all have their unique aspects. Things to take into consideration:
O2/Voda use 900Mhz so they go through walls better on 2G
All networks are equal when it comes to 3G signal on a technical level, it's just down to footprint/number of masts.
3 Use Orange for their out of range 2G network
3 Also have a network site share with T-mobile
Virtually all the networks use different HSDPA equipment & speeds (although it's pretty universally known that o2 had the slowest HSDPA deployment, Voda have the fastest and T-mobile the largest footprint)
Orange cover the most "people" but Three cover the most territory
Voda support pico cells, Orange support UMA for some models.
If you ever wanted to know more about your signal / towers around you I'd give the ofcom sitewatcher site a visit or if you have an android phone you might like Antennas:
http://www.androlib.com/android.application.com-technolatry-antennas-qiEq.aspx