One2one
o2
Vodafone
I started with one2one in 1997. They had 80% coverage then and were pretty bad. Luckily, I had an almost full signal at my house. I swappped to Orange a couple of years later because most people I knew seemed to be on Orange and cross network minutes hadn't come along at that point. Went to o2 in 2005 and I'm still with them now. I did have a phone with Vodafone for about 4 days in 2006 but returned it. They still owe me £50 and would admit this everytime I rang them but the cheque never arrived. Because "it takes 28 days to process", I could only ring once a month to see where my cheque was. I gave up after about 10 months.
So I wouldn't have Vodafone again and I wouldn't have 3 based on friends' experiences with them. I've heard a few stories about trying to cancel at the end of the minimum term. The "we never received the letter" is popular for 3 but my favourite was a friend calling 3 to cancel and 3 asking for this in writing. Ok, pretty standard with mobile contracts so she sent a letter of cancellation. The phone had been replaced with another network/contract by this point so wasn't in use. 3 continued to bill her so she called up.
Friend: Why are you still billing me? I sent you a letter of cancellation.
3: Yes, we received that
Friend: oh good, so it's cancelled then?
3: No
Friend: why?
3: Well, When we receved the letter, we tried to call you to make sure that you wanted to cancel
Friend: Right, OK?
3: We couldn't get through (phone was off/out of use by then) so we didn't cancel "just in case"
I'll say one good thing about 3. They really upped the ante when it comes to included minutes. Up untill then, 200 minutes was pretty standard for £30. 3's main tariff was 500 minutes for £25. It wasn't long before all the other networks started offering better deals for higher minutes. At the time, the 'o2 max' tariff was £75 for 750 minutes