BIG OL' DISCLAIMER: I work for one of them. I'll be objective. Like Fox is about BMWs 
There is no overall best network. There is the best network where you are often though (town/office/commute/place of drinking)
You mean Hutch, that awesome HK Company? http://en.wikipedia.org/wiki/Hutchison_Whampoa . Ah back in the day when they owned us Hans did some funny stuff. They still have the upside down elephants in meeting rooms from his fueg shi advisors or whatever it's called.
Lets clear it up:
Today
3: Have access to their 3G network and T-Mo's 2/3G network (they used to use Orange's 2G network until last year)
Orange: Have access to their 2/3G network and T-Mo's 2G network
Voda: Use their own network solo
O2: Use their own network solo
T-M: Have access to their 2/3G and 3's 3G network.
By the end of this year
3/Orange/T-Mo will be on one giant network. Hopefully.
O2/Voda will be solo (unless they actually do the share)
o2 does get a kicking often for data because they kinda neglected HSDPA which it turns out - was kinda crucial to the telcos for data expansion that they were forced to do 2 years ago. All networks spend a lot to keep up with current data growth but if you start behind it's harder to look good when the competition is beating you with a stick etc... They honestly aren't that bad.
"Largest network" is hard to qualify. For example AFAIK Orange still have the largest population coverage, 3 have the largest geographic coverage. While it's true, 900Mhz signals go through walls better and travel further they also only use this for 2G - 3G is currently always at 2100Mhz for all networks (aside from 900Mhz 3G re-allocation trials, I'm choosing to ignore this as rollout is bad).

There is no overall best network. There is the best network where you are often though (town/office/commute/place of drinking)
With the money that the UK government is pumping into 3 at the moment (to ensure a British owned company has the best penetration), I'd say 3 will be, even if it's not quite there now.
You mean Hutch, that awesome HK Company? http://en.wikipedia.org/wiki/Hutchison_Whampoa . Ah back in the day when they owned us Hans did some funny stuff. They still have the upside down elephants in meeting rooms from his fueg shi advisors or whatever it's called.
Lets clear it up:
Today
3: Have access to their 3G network and T-Mo's 2/3G network (they used to use Orange's 2G network until last year)
Orange: Have access to their 2/3G network and T-Mo's 2G network
Voda: Use their own network solo
O2: Use their own network solo
T-M: Have access to their 2/3G and 3's 3G network.
By the end of this year
3/Orange/T-Mo will be on one giant network. Hopefully.
O2/Voda will be solo (unless they actually do the share)
o2 does get a kicking often for data because they kinda neglected HSDPA which it turns out - was kinda crucial to the telcos for data expansion that they were forced to do 2 years ago. All networks spend a lot to keep up with current data growth but if you start behind it's harder to look good when the competition is beating you with a stick etc... They honestly aren't that bad.
"Largest network" is hard to qualify. For example AFAIK Orange still have the largest population coverage, 3 have the largest geographic coverage. While it's true, 900Mhz signals go through walls better and travel further they also only use this for 2G - 3G is currently always at 2100Mhz for all networks (aside from 900Mhz 3G re-allocation trials, I'm choosing to ignore this as rollout is bad).