Which is the best duo for say 50+

[Loveless]

I don't think you'll be too bored duoing with DDs. Once you pick up a good rhythm it'll be mob after mob after mob with nice exp. And although they can 'tank' up to a certain amount you'll still have to keep a watch on their HP. Also, the good thing about being with a mage is that they have the AOE spell which have a lesser cooldown (I think? At least it seems faster than my AOE spells) so if they are able, you can go through 2-3 mobs at a time.

Time to time you'll have a surprise mob behind you and all that excitement. :P

[TheGhanExpress]

it depends on what you're killing, i'm only level 40 but mage and cleric are great. if the mage is hitting too hard for cleric to take agro the mage can tank and use magic burst to keep agro with great efficiency.