I will say sky and I have my reasons.
Sky built their network to handle unlimited usage from day 1, as such it has massive capacity, as well as excellent peering.
Whilst BT and talktalk originally designed their networks with a policy that limited p2p etc. But then later for marketing reasons adjusted to unlimited usage.
I would stay away from plusnet, although kitz accepts plusnet's stance its a BTw issue, I disagree with her, people have moved to other BTw shared isp's and resolved their performance issues and plusnet staff are no longer blaming BTw. BTw may have issues on their MSIL's but I dont think they solely to blame for plusnet's problems.
So ideally you want to avoid BTw altogether which means talktalk or sky, if you willing to risk BTw then use BT broadband not plusnet.
Also for what its worth I would probably do 40/2 and 80/20 also if I was an isp, purely to maximise revenues, although I would perhaps considering doing a 40/10 in the middle. I think what killed 40/10 on plusnet was their daft idea to do 40/20 which probably murdered their 80/20 signup rates.