Really there's only one class that every raid force can't get enough of: Rogues. You can never have too many Rogues. Clerics come in a close second (you only very rarely can have "too many" Clerics), but I'll come back to them.
To a lesser extent the same is true of every DPS class, it's just that Rogue tends to be the highest and most consistent source of DPS. Less so in Classic maybe, but it becomes more true as the timeline progresses ... ie. as raiding becomes a bigger and bigger thing. But still it's not like any guild will say "we don't want more DPS" ... it's just if they already have a few Rangers or Wizards, they might pass on a new one. But Rogue #12? Welcome to the guild.
As I said Clerics are almost the same: you
can have too many Clerics, but very few do. Most guilds in fact "repurpose" other classes as Cleric substitutes: Druids (subpar Clerics due to their worse healing spells), Magicians (mod rod makers to give Clerics mana) and Necromancers (ie. "Twitch" mana batteries for Clerics).
Also once Kunark comes Shaman with Torpor will join that list. I almost feel like Druids are the least desired out of any of them though. Clerics/Magicians/Necromancers all contribute to the CH chain, whereas the Druid/Shaman mainly help with healing outside that chain ... and a Torpor Shaman can do so more efficiently (plus they can slow).
So anyhow, one way you could interpret all of the above is that I'm advising you to go make a Rogue or Cleric (or to a lesser extent, another DPS class or pet Int caster). But another way to look at is ... Warriors, Paladins, Shadow Knights, Bards, Enchanters, Wizards, Druids, and (pre-Torpor) Shaman ... they all have a role on raids. Just because that role can't use five of the class, that doesn't mean guilds don't want
at least one (if not more, both because 2+ might be valuable, and because usually any given player won't show up to every raid, so you need redundancy).
Heck, in my experience every guild wants at least one of even the most (perceived) "useless" class ... just because no one likes to see loot rot
[You must be logged in to view images. Log in or Register.] So really what I hope you'll take from the above is that
every class has a raid role, and you should pick the class that appeals to you, then pick a raid guild that needs that class, rather than pick a class based on perceived need.
But if you want to be able to join
any raid guild, pick Rogue or Cleric, and definitely don't pick Druid (for that reason at least).