We use AAISP here, on a fairly decent (will sync at 17M) BT 21CN ADSL2+ line (we're not big downloaders, so can't really justify the extra cost of a Be line). We pay for three of their confusing pre-pay units per month (£23 month all in, which I consider to be cheap as chips), which easily covers the 3-5gig of peak-hours usage from general web, mail, interactive shell and so on. (Bear in mind I'm at home most days and barakta only works 4 days a week, so we're around more between 9am and 6pm than you're likely to be.) I have a some traffic shaping at our end that restricts download to just-less-than-you-need-to-comfortably-stream-video during peak hours, but in practical terms that's more about preventing errant Windows boxes downloading hundreds of meg of updates on the sly than keeping us from getting sucked into youtube.
After 6pm, it's effectively all you-can-eat downloads: bit of scheduling in your bittorrent and iplayer clients and you're sorted.
Being AAISP, the only restrictions on throughput are those caused by BT (usually line problems and congested exchanges, which A&A will not hesitate to report to BT as faults, with the expectation they should be resolved). There's deliberately no contention at their end. The only traffic shaping they do is prioritisation of small packets and throttling to 95% of the BRAS limit (which you can enable from the control panel) in order to keep VOIP-like streaming protocols performing well even when the line is saturated by downloads. Their killer feature - the continuous quality monitoring graphs - means you can actually prove that your line's performing as it should using SCIENCE, rather than having to infer things from your router's sync stats and speed test sites.
They'll give you all the static IPv4 and IPv6 addresses you need for no extra charge. They've also finally managed to obtain some native-IPv6-capable consumer-grade routers, so you can use the current version of the internet without having to be a Linux geek.
I don't think their reputation for customer service needs elaborating, but not only are they happy to deal with deaf people (or the landline-impaired) by SMS, but it's entirely normal for the director to randomly appear on IRC on a Sunday morning at mention of a line fault, and take a break from watching the F1 to do the relevant BT-kicking, even though their support is officially business hours only. What's not to like?
If you're not going to be leeching during business hours, the only sensible reason to go with anyone else would be because you've got a choice between 10km of crackly aluminium wiring from hell or a perfectly good Virgin cable line.
If you're feeling cheap and leechy, the good ISPs seem to be: Be retail, Zen, Eclipse, Plusnet