Good 5 man comps purely for pve / dungeon leveling

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The above responses are really good. I would not recommend a melee team for your first team.

If you are only having one single group to cap I would do:

Tank+2Mages+Warlock+Healer

DPS being ranged is much easier overall than melee/hunters and 3 Mages, 3 Warlocks, or any mix can work. Mages are best DPS (but more squishy) though many people really value having 1 Warlock in the party which is why I recommended that comp above.

Any Tank (Warrior, Druid, Paladin) works fine for Dungeons. Warrior is needed if you think you want to try to be a Raid Tank sometime and likely the best for a single Tank role in a dedicated party though I personally prefer Paladin or Druid as they can both serve multiple roles and I especially prefer Paladin for Dungeon Tank.

Any Healer (Priest, Shaman, Paladin, Druid) works. Priest is the proto-typical and best overall though any can work if you want one of the others to be your main for raiding or what not and/or need something besides a priest for some reason.

With all the above in mind, if I was forced to do a single set of 5 characters to cap with no other characters ever, for my goals I would do:

Paladin+2Mages+Warlock+Druid

Best of luck to you.