A word, first. The following discussion would apply equally to a non-IK barb that used any of the traditional combat skills (... as I have no experience of a throw barb, a double throw barb, or a singer).
The "best" setup if there was just one, would be ridiculously expensive to make - and not practical for most players. But there are some very good, very affordable merc setups available. One such setup, (everything costs Pul or less, I believe).
Might merc:
Vampire Gaze (high LL preferred)
Shaftstop
Reapers Toll
Capable of soloing hell ancients. Some comments about it...
1. High survivability when there is any chance to leech and the elemental attacks aren't overwhelming. It isn't a merc to take to Travincal. This merc will not die too much (or ever!) if you take some notice of him. I spend a lot of time running the pit, and he absolutely never dies down there.
2. Huge damage. His might aura is more useful to him than to you. It means a lot of extra leech for him, but more like a little extra for you. However its the following fact that really makes the damage huge...
3. He is a walking talking curse. And decrepify is absolutely the best curse for barbs 99.9% of the time. It doesn't kill *quite* so fast as amp, but it completely nullifies the offensive capabilities of most critters suffering it (hence capable of soloing hell ancients). Decrep removes a lot of physical immunities too. Once the melee starts expect to wait one good jab for decrep to kick in and expect it to finally cease when the last enemy falls.
A BotD merc probably kills better of his own accord (faster weapon, higher damage!), but the decrep massively increases your killing power, which is a far better/faster way to rack up kills.
4. As much as you can, upgrade and get ethereal versions of these pieces of equipment. I've never had the chance to see what difference an eth reapers toll would make, but I expect it would be quite noticable.
5. 3 sockets to fill. I don't know the bp's for reapers toll, so I don't know if shael or an ias jewel would be of much/any use. But an Amn never hurts.
As for the armour and helm, at least one of them needs to be used to make up resistances. Um is expensive, but other runes can do a single res (think fire or lightning), and that might be more useful (cold and poison barely matter, imho). Also think about using jewels. Otherwise so-called junk jewels might be a very effective way to socket in some extra reses, and perhaps other nice mods.
Really, the only *key* piece of equipment in this setup is the Reapers Toll. I'm probably going to try removing the shaftstop and trying one of the runeword armours that offers some resistances on one of my Reapers Toll mercs. Remember with this setup that its not worth anything if the decrep is constantly over cursed (so don't use saracens chance, for example).
If cost was no object, I'd probably want to try something like:
Ethereal CoH / Eth Reapers Toll (Amn) / and some hat.
That new runeword hat with holy shock would be interesting. A very nice place to pick up ~500 lightning damage per hit. Even Andy's Visage would be nice for the skills and leech (just as long as fire res doesn't kill him). He might also appreciate a CoA with a couple of shaels in when he's being ping-ponged by Pindles minions or snakes!
Cheers.