This site was started due to dissatisfaction with other guide compilations, which generally seek to host proprietary guides for all classes on one site. Some of these websites are good jumping off points, but, in general, they suffer from being out of date or inaccurate for at least some of the guides. That being said, the best are listed below.
http://www.icy-veins.com/wow/class-guides – A very reasonable starting off point for people new to raiding. The Icy-Veins forums also have bits of useful discussion.
http://onuraslan.github.io/SummonStoneMirror/summonstone.com/ – A mirror of the now defunct SummonStone website. This project had promise, but was closed by its creator, Skullflower, a notable Death Knight theorycrafter and raider, when he took a job working for Blizzard. Nice guide format, a better jumping off point for some classes than Icy-Veins.
http://www.mmo-champion.com/forums/247-Class-Discussions – The MMO-Champion class forums have the best theorycrafting available for some classes. Other class forums may be inactive or, worse, have inaccurate information.