Not sure what they are called in D3, always think of D2 and refer to them as Mercs.
In D2 there was some definite correct choices when it came to Merc as an Act 2 merc with a specifc RW for your light sorc etc.
What about D3? I pretty much only use the Act 1 guy for heals and resource regen, is that what is generally used in HC? Im sure there are builds that could use other hirelings, but in general does everybody use the meat shield?
Templar is great in that he has heals (which have effectively saved me from triggering my fatal passive more than a few times). He also has a stun attack that comes in handy. I try to set mine up with Eun-Jang-Do dagger, Freeze of Deflection Shield, Ess of Johan amulet, Occulus Ring with double IAS (can stack to 18%) and a Unity.
The best use of a follower, especially in HC, is not damage. Rather, you want crowd control mitigation. A Templar set up like this (at least in 2.2) can darn near perma-lock a RG once he gets below 20% health.
There are other CC set-ups you can use. Such as Wyrdward ring with Thunderfury, or you can stack up DPS if you like, to some effect.