Raids need tanks, healer, dps and support.
The best tank class for raiding is the warrior. Followed by shadowknight and paladin.
The best healer class is the cleric. Followed by shaman and druid.
The best dps class is wizard. But also providing excellent dps are magician, necromancer, berserker, rogue, monk, ranger, beastlord.
There are no "best" support classes per se, it just comes down to what the remaining classes have to offer. Enchanters provide mind buffs, can boost dps of other classes and have some decent dps of their own, but not enough to put in the dps class. Bards offer excellent abiilities to improve the capabilities of everyone else in the raid. Bards and monks are excellent pullers, but pulling has become an ability that several classes can now manage to excel at, especially when the mage's call of the hero spell is added to the mix.
Several classes have a buff or two which are very useful to have cast "MGB" (Mass Group Buff) on the entire raid but for some ONE of that class is sufficient to keep the raid buffed. Ranger, paladin, druid, enchanter, beastlord all have those kinds of buffs. Every raid wants at least one player to be able to contribute these buffs to the raid.
Frankly, if I had to pick a class that can most be left out of raids, it would be the necro. Necro dps, based on dots, works well on boss mobs, but even then you can almost always replace that kind of dps with more wizards and mages.
If you want to have the easiest time joining a raiding guild make a warrior or cleric or wizard. If you want a challenge, make a necromancer,
Yes, my comments are arguable. I look forward to the arguments.