A lot of different parties might work well in the WLD, so for the most part, you're free to play whatever you might like. I have read through it, and I can offer a few suggestions. First of all, someone good with traps will be really, really helpful. A high Search skill and a high Disable Device are both needed. The book actually suggests not allowing rogues to take 10 or take 20 on searching for or disarming traps, so be prepared to get your nose caught in a few traps here and there, especially early on. Secondly, don't neglect to have a character with some social skills (bluff, dipolomacy, etc.). There are actually quite a few factions and groups you can ally with, and several tough fights than you can actually talk your way out of (the Dungeon has more than just mindless monsters in it, after all). Finally, try and build your group so that your characters can work for quite a while before needing to rest--there are very few safe places in the Dungeon.
I'd say that the druid might have a hard time of things, unless your DM alters the Dungeon a little. As is, you can't summon creatures in the Dungeon, and you'd have a really hard time replacing your animal companion if it died. Plus, many of the druid's best spells are useless in most of the Dungeon. There are few spellbooks and scrolls in the Dungeon, so a Wizard might have a hard time of things as well, unless your DM gives you an alternate way to get spells (in which case, a Wizard might become quite helpful). Ranger is probably not a first-tier good choice either, I'm afraid.
With all that in mind, here's my class/race suggestions for a group that has a lot of options and quite a bit of stamina:
1. Dwarven rogue: the extra hit points will help you survive the traps you can't disarm and since most of the Dungeon is made of stone the Stonecunning ability will really come in handy with Search checks. And they can fight, too, although they should always be working with another party member to flank opponents.
2. Human Warlock: Warlock are as good as gold in the WLD, because they don't need to rest to regain their magic. In fact, Warlocks are so good, your DM may not allow them. Do keep them out of the front lines, though. The Human's extra feat will help you get Precise Shot as early as possible (Warlocks need Precise Shot).
3. Elan Psychic Warrior: A psy-warrior can deal some good damage, and has quite a bit of versitility in terms of powers and abilities. Elans are not only naturally psionic, they can go without food or water for long periods of time (thus conserving the party's resources), and they can spend power points to reduce damage taken. A flexible mix of offense and defense. (the main weakness of this build is they you're always going to want more power points--pick your feats accordingly).
4. Whisper Gnome Ninja: The best possible scout, and a decent melee character, in a pinch.
5. Human Favored Soul: I'd go with Favored Soul instead of cleric because of the slightly higher spells per day. This will be your healer and your diplomat--spend your extra skill points on Diplomacy and make sure to pick up Augment Healing (every little bit counts).
6. Illumian Fighter/Cleric: Take the 'Krau' symbol and the Practiced Spellcaster feat so your caster level stays high. Get the War domain, if you can, for Weapon Focus. You'll make an excellent backup healer and tank. If you take the 'Aesh' symbol at second level, you don't have to worry about having a high Wisdom.
I think s/he should give you some guidance here.
