thorns and IM

[Crystalion]

A heavy summon build would profit most from Blessed Aim merc, because the Skeletons do lots of damage, but have bad AR in higher difficulties.
The Might hireling is the better choice, if the build has some other AR source handy (say, Passion or Whisp Projector, or - best of all choices, but very rarely to get - Beast).

Enchant.

Massive +%AR.

Demon Limb or Enchantress.

Did I mention Enchant yet? ;)

(if you can count on the AR%+ to your skelies then a Might merc probably wins out over BA).

edit: okay, okay, fair is fair... if you can count on Conviction and/or Inner Sight at high levels, the AR issue also goes away (for example, if you see a vengence/conviction paly using a rogue merc, you might want to invite them to join your party, as whiffing is going to be a rare event).