Im sorry, but its a fact that vodafone and o2 have the least 3g coverage out off all the networks, you only have to do a bit of research to find that out. So dnt absolute bobbins me.
As for orange, there recent internet or lack of it is all over the android and 3g forums, do your research! Ok maybe some of the problems are to do with the orange/T-Mobile merge but theres a lot of customers complaining of problems with their internet on orange handsets in recent months, especially on the Android forum.
3 radio share with T-mob on most sites outside of the core, self owned city based infrastructure but this makes them pretty polar - in the city and you're fine. Step outside of one - you're definitely not.
I work out in the sticks and still get a reasonable 3G signal on a Three dongle, ...the vodafone customers have a better signal out there yes ... but its 'GSM' (no 3G coverage out of the towns/citys) meaning dial-up type internet speeds of 40k.
The fact is I have sim cards for Three, O2 and Vodafone here and travel around a lot, and from my own experience Three have the fastest most reliable internet speed on my handset. Sure Vodafone have better coverage if you include their 2G coverage, but the OP's question was about internet, so we may as well discard 2G as its painfull even browsing over it. I would say T-Mobile is the closest
providing you buy your contract direct from T-Mobile AND get the Web N Walk Plus with 3GB FUP. If you buy from a 3rd party like CPW or have a tarrif that doesnt have the 3GB FUP your speeds capped at 350K!
http://blog.three.co.uk/2010/07/19/our-mobile-broadband-tops-latest-independent-yougov-survey/
The YouGov survey asked over 2,000 customers from Three, Vodafone, Orange, O2 and T-Mobile and some smaller operators to rate the level of service they got from their provider on twelve key points. These were installation of software, network coverage, getting connected, staying connected, reliability during the day and during the evening, download speeds, upload speeds, ease of use, customer services, billing and overall quality.
You rated us the best in ten of these twelve areas and in feedback comments, you also labelled us as the best value for money operator, and you gave us the highest rating of user satisfaction.