Shaman: slow, buffs, debuffs, root, pet, heals, regen, DoT spells, many tools to maintain mana pool - I believe the most common necro partner.
Druid: regen, heals, ports, damage shields, tracking, harmony (outdoors only I think), attk debuffs - A viable choice
Bard: regen, some slow, mez, harmony, run speed, aggro drop (fading memories) - can only be played to 100% if you have a tank or tank merc as well.
In my experiences, a Shaman or Druid is the most common partners for a necro. The built in heal in the endgame shaman slows helps offset some of the issues of trying to make pets tank. Their spells are aligned well with necro spells and the two can do a lot of content without other players.
Bards pose an interesting alternative but require a tank in the group as an npc will always attack a player in melee range over a pet. Their slow is not as strong as a Shaman (or Enchanter) but they do add the ability to mez level appropriate content (single or AOE).
Just my 2cp