If I were making the same decision, I would be looking at which class would most increase my capacity to be independant i.e. do what the necro cannot. I would also look for automated ease since you will be boxing.

Wizard- huge dps at mind boggling speeds and travel/escape. Easy key mashing as a box, nothing complicated. Nothing there that a necro cannot do and take a little longer.

Shadowknight- Good tankage and uber aggro. Unfortunately you need something to both keep it alive, and slow mobs it is attacking. Your necro cannot do this. In my opinion, a SK would be a liability.

Magician- Decent tankage via pets and good dps at high speed. Self sufficient at keeping pet alive and end-game uses raging/rampaging servant pets that are incredible. Ding-ding-ding-ding! This would be my choice.

To recap- the wizard would greatly increase the speed at which you can kill mobs you can already solo with the necro. The SK would be a massive liability, requiring you to find a healer and slower - not to mention consuming 95% of your attention keeping it within melee range, using abilities and aggro spells, etc.

The mage would actually broaden your capacity. Mages solo summoning mobs like your kid brother solos M&Ms. They are also great kite partners with the pet, easy key-mashing setup for boxing, etc. They also can increase your mana regen via mod rods, and give you unlimited supply of pet equip/toys.

Your call - you are the one that will be driving!

i think you should try to determine excatly what you want from a secondary character (what role you want them to fill, and for what exactly, and also look at what other unique abilities that class has to offer like snare, evac, etc)

.. to do that you should first determine what you already have, as a necro (at least this is how i like to break things down)

as far as roles go in groups i see them as: DPS, tank, healer, puller, slower, crowd control

as a necro they mainly fill: DPS, and pulling. necro's can snare and kite, which makes the wizards kiting role less valuable for example. By seeing what the necro can do, you see what other roles need to be filled and you can choose as you think would be most beneficial to you.

you have to ask yourself, do i want a tank? or do i want more dps?
as far as a tank goes, they take a long time to become strong.. very AA/gear dependant. On the other hand, the magician offers a pet which can tank many zones, and it dosen't require AA or gear before it becomes useful.

from the classes you listed (sk, mage, wiz) i think Mage would offer you the most versatility. you could even kite mobs and have the wizard use a backstabbing water pet if you didn't wanna have the mage pet tank. Wizards are nice though, i have one, always nice to have ports. I also like my wizard because he was doing fairly decent damage even with low AA and non-raid gear. in the end it's just a matter of preference.

If I had a 2nd computer/2nd account lol I would deffiently chose Mage.

Reason: Pet Toys, CoH, Best pets, and Decent spell damage.

Not SK just because he would have to chain Potions and Lifetaping and not a Wizard just because I would much rather have the mages utility.

I do a 71 mage/ 66 necro combo, and its awesome!

i duo alot in WoS which is pretty easy with the snares and the mages pet tanking