In order of "best" to worst" from my experience:
Vodafone - Good reception in most places including international roaming, reasonable packages (especially GPRS, which I use often) plus good range of phones (and if you avoid buying from them directly, you can get unbranded ones, which is the better choice). Rather poor customer retention.
Orange - Would have topped my list for best network but their poor range of phones put me off everytime I look at their network... this is then followed by Orange branding. Their customer services is fantastic though (in my experience).
O2 - Bad in the past, starting to improve but their customer services have been poor for a number of years. Good coverage and tariffs, especially for students.
T-Mobile - Poor coverage, poor range of phones and as of my last upgrade with them, they don't seem bothered with existing customers AFAIK. Still using an old tariff with them since their new tariffs are rather poor.
3 - Oftel should consider getting rid of 3. Customer services doesn't exist with call centres based in India (apart from their Scotland branch, which is rather difficult to get to, if they still exist). Took me nearly an hour to retrieve a PAC code for my sister and was misinformed about the PAC code process by 3 different people who couldn't understand my fluent English (they try so hard to put an American accent on, why do they bother since it's so obvious

). Service isn't great, range of phones are starting to improve, tariffs are cheap but who would go for a network with such ridiculous customer services?