It depends what you want it for... You get what you pay for really....
For fast d/loads you usually have to suffer with a cap on how much you can d/load each month.
For unlimited downloads you usually have to suffer with a slower connection.
Most ISPs practice 'traffic shaping' as stated in their 'fair usage policy' - always check the T&Cs. And a lot have peak & off peak hours, with some offering unlimited downloads during off peak hours which may suit you.
If you want to uses P2P file sharing programs you should look for an ISP that doesn't have a fair usage policy, or one that suits your needs.
If you don't use the internet that much you should be OK with the cheaper providers such as talktalk, wanadoo, tiscali etc but if you are a serious internet user those ISPs are 1 million miles away from what you need!