Depends on what your goal is. Most people are going to tell you Cleric, but Cleric+Ench doesn't shine until you start doing dungeon camps, I'd say starting in the lat 40s?
Really, any class that can Snare or Heal will bring just about all that Enchanters are missing for abusing the game. Druids and Necros shine the most while Exping (IMO), but either choice will be fine.
Druids will let you travel Much, much easier thanks to SoW and Ports, and gives you better buffs for yourself/pets. Druid+Ench+Outdoor zone=Nothing can stop you.
Necros allow for potentially more DPS and has a wider choice in charming, as there are many, many times more Undead than Animal charm targets. They have better Damage options that Druids, but its a moot point as you're charm kiting...
Necro honestly doesn't bring as much to the table as Druid in the 12-40 range, though. Enchanters have a better Fear than Necros for a very long time(Ench's Chase the Moon at 16 [6 ticks] versus Necro's Invoke Fear at 34 [7 ticks]). Druids bring Snare and SoW, so you'll almost never get too hairy, and get Root much much much earlier than Necros. Harmony gets rid of those rage-inducing critical resist on lulls. Even in a dungeon, Druids will bring spot healing and evacs.
Honestly, when I play my Enchanter (37 or 38 right now, can't remember), I /who all (level-range) Druid first, then look for Necros, Then look for anything else/a group.
That being said, Any class can duo with an Enchanter reasonably well.... Warrior is probably the only class I wouldn't do, but then you just need a Cleric and you've got the holy trinity on your side.
If you did plan on getting 50+ I'd probably echo Ahmet and say Shaman+Enchanter, that way either of you solo would be able to take on the crazy camps that Druid can't do. I imagine this is the trade off for Necro; Druids rock at making "lazy" plat -- selling ports, Powerleveling, camping AC/Hadden/Money outdoor mobs) -- but eventually Necros are pretty close to Enchanter and Shaman on amazing soloing abilities.
Edit: Can anyone besides Necro/Shaman/Enchanter handle Seb/HS named camps, even?
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Some kind of... Bubba Ho-Tep?
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