Rest assured there have been significant changes to the game to make it more playable and fun.
The guilds have had a long time to level, and you will find a few uber guilds on almost every server. Raids have been added, fixed, and updated, so there is plenty to do.
A few class extremes follow:
Fighters: Guardians have the best defense but the worst DPS. They are particularly good at standing there and taking a pounding while the rest of the group kills the mob.
Healers: Templars have the largest single target heals and can wear plate armor. Like the fighters, they have low DPS.
Mage: Wizard has the largest single nuke which does something like 4K damage to a single target.
Scout: An Assassin would have the single largest stab, but a Ranger is best with a bow.
Now to the always present question of what's the best for a raid? Well, a solid mix of everything is important. You'll need fighters, healers, plenty of DPS, and of course, a Mezzer.
As you stated, the uber raid guilds may guide their recruitment by selective slots. Or maybe they are not taking new applicants. It really depends on each specific guild, and they all run a bit differently. Some guilds may not even look at you until you are level 60 - a significant time investment only to find out you are not what they are looking for.
I don't really know how to recommend a particular class that would guarentee your placement. Even if I could, that guild's requirements may change by the time you reach a significnat level. :(
I guess the best advise would be to select a class you enjoy, and go from there. By getting your gear and abilities to the best available, that would raise your stock when it comes time to get into a guild.