From my personal experience as a Necromancer - It doesn't really matter too much what pet you use. Except the Undead Knight (which I don't understand, too much), for some reason people seem to dislike it. As for the literal Reaper - I don't use it at all. But it is worth to upgrade the spell at least.
Max your reaper for the buff to normal pets. Nightshade (which will show as undead knight, unless recently fixed) will do far more dps than the old 2 0 pets do.
If you care about maximizing your DPS in any way, it ABSOLUTELY matters which pet you use: Reaper - Get your spell level up just for the buff it gives to other pets, but don't use it for current content. The Reaper tops out at lvl 110 and gets resisted far more than the current pets, so it does significantly less damage. Tank Pet - It's a beast for tanking, so it's definitely useful. However, it's usefulness is extremely situational, so you should only be pulling this one out every once in awhile. Mage Pet - It IS possible for the mage pet to out-parse the scout, but only when you have a ton of adds. Even then the difference isn't very much, so it's hardly worth it. Scout Pet - Use this 99% of the time.
