Depends where you live.
Some companies can be reliable in some places and not in others.
What I will say is don't go sky regardless, if it does go wrong you will be dealing with the thickest most useless bunch of morons alive. I don't expect call centre workers to be genuises, if they were techies they wouldn't be working in a call centre for £7 an hour but I expect them to understand basic terms and reasonable diagnosis. I also expect them to know what they are contractually obliged to do (it's a standard broadband contract, it takes 5 minutes to read).
They'll say "Oh it's BT's fault" or make up some bull**** excuse "Oh must be a bad router" or "Some new microfilters" like I wouldn't think to swap those out, and if I tell them that they'll say I have to try new ones anyway and refuse to help me, had nothing but problems with sky it still gets 256kb/s speeds tops and cuts out several times a day, bloody useless.
Plus when they did finally send an engineer out he blamed it on the extension line I installed, which was already quite obviously disconnected and not in use any more, he checked the connections and then went home without actually doing anything. Bunch of cowboys that lot.
I find Plusnet to be quick, reliable and the people in the call centres are friendly and helpful. But it does have a monthly cap, which is not good if you are streaming. If you use streaming services sparingly and don't play online games that drain your download limit (I play WoW and it is hardly noticeable), it is absolutely fantastic. Caps of 10GB and 60GB at the moment, with special offers.
Totally agree with the above about Sky though. Quick to send letters telling you the cost is increasing or you've gone over your limit, but never helpful. Never.
Despite the previous two posts, Sky Broadband is actually quite good. The link above shows its one of the best rated major ISPs. Also if you go for the unlimited package (£7.50 if you have sky tv or £10 without) then its fully unlimited without traffic shaping, not something that Plusnet/Virgin offer for the same price.
Plusnet seems to be the cheapest just from adverts that I've seen, but then you need to make sure it's actually decent. Read some reviews online and stuff. I'm with Virgin and although it's not cheap, you actually get what you pay for. We're supposed to get 20mb, and on regular speed tests I get 19.5mb. My dad's with Sky and he gets about 12mb of the 20mb he pays for.