For melees a shaman is a better partner, frankly. For casters, however, I'd go druid for the mana regen and better heals.
For hybrids, pick one.
On the hijack subject, mage/necro is a very good, fast combination but there is very little room for error. I duo my mage and cleric quite a bit, and except for the limited CC (cleric root), it's a good combination as well.
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