The dedicated melee group would be:
Warrior, Cleric and Shaman
The warrior is The Tank, especially later on, the cleric will keep them all up and walking and provide things like Calm, the shaman is slower, buffer, DPS and secondary healer rolled into one.
The variant for more active players:
Shadowknight, Cleric, Beastlord
The pallies and SKs suffer as tanks, when it comes to raid mobs and high end encounters, but fpr group action they are fine, and offer usually more fun than a warrior. I choose SK over pally any day, as he can single pull in several ways and is better with DPS. Cleric is more or less a given, the king of the HP buff, the master of the rezz and the best healer. Beastlords can replace shammies/chanters for slowing and are better DPS. Less solid for crowd control and slows than the other two, but still a viable choice, unless you attack high end mobs.
The other option for the control slot is the bard, but I hate them for personal reasons, and they are difficult to master (i.e. be effective and not a walking TPK).
A group of casters could be:
Magician, Druid and Enchanter/Necromancer
Many mage/druid teams are around and are pretty efficient. So you only have to add a complementary class. Chanter is OK for slows and crowd control and his other stuff (he could charm, help fearkiting, etc.) is not bad, if he knows his job (not the easiest class to play). A necro is also a good addition, bringing a pet and murderous damage into the equation. Druids are the allrounder class, being able to heal, nuke, buff, track, and have nice travel spells.
My preferred full group of six (yes, you dont have six, but anyway :) )would be:
Warrior, Shadowknight, Cleric, Shaman, Druid, and Rogue.
In this group you have several redundancies and everything you need, even if one or two go LD. The warrior is primary tank, the SK puller and secondary tank, the rogue is the DPS and scout, the three WIS casters have all the nice buffs and every heal there is. Additionally they have the travel, rezz and utility spell lines. Theoretically the SK, druid and shaman can all have pets active, and all three have different DoTs, which does add to the total DPS, which is less than it could be in a high damage group, but works for everything a single group can tackle.