I agree with much of what Angelus Censura said. Zerkova is generally believed to be Khador's weakest caster, though in her defense she has some tough competition. Karchev is kind of a one trick pony, in that the "power slide" of delivering your heavies is the only thing he does well (though he is extremely cool looking). Strakov's biggest strength is his blatant alpha strike capabilities (Overrun, Superiority and his feat), but Occultation is also a neat spell and certainly something different for non-Kayazy to have (though often best on him for added survivability).
Here're some models that are considered great with pretty much any Khadoran caster:
Widow Makers - the Sniper rule makes them an excellent anti-infantry tool and can help them take out systems on damaged jacks easily. The Widow Maker Marksman solo commonly accompanies them, as they want to stay as far away from their targets as possible and he gives them the ability to move 2" after killing something. Also the mercenary solo Kell Bailoch is often there unofficial unit attachment, as he does exactly what they do, only with higher RAT and shoots twice.
Winterguard + Kovnik Joseph Gregorovich - often referred to as the Winterguard Deathstar, it's a full unit of Winterguard Infantry with the unit attachment and the full 3 rocketeer weapon attachments. With Kovnik Joe to give the whole unit attack rolls, they become excellent and wiping most single wound infantry off the board. The UA allows them to use the Bob & Weave order, increasing their DEF, as well as giving them the option to fire pow 10, 8" sprays rather than the default single-shot, pow 12 blunderbuss. With certain warcaster's buffs (the Butcher's or Epic Sorscha's feat for example), they are also able to put some serious damage on hard targets as well. While they're on the advance, Joe can give them tough and fearless to help more of them get into spray range (though the rocketeers have a very respectable range at all times). The Winterguard Rifle Corps can serve a similar function at a longer range (and can benefit from Joe's abilities), but the Infantry are more numerous and well rounded.
The Great Bears of Gallowswood - a 3 man unit of incredibly high MAT character weapon masters. Each one individually would easily be worth bringing as a 2 point solo, so their 5 point cost is a steal. In short, they can seriously wreck face and are just a generally amazing unit.
Ironfang Pikemen - more specifically with the Black Dragon unit attachment. Ironfangs have always been good (reach, CMA, high base pow, crit knock down), but as with all melee units, they're no good if they can't close with the enemy. The BD UA gives the unit a mini-feat (Iron Zeal) that gives them +4 ARM and immunity to knock down, making them significantly more survivable while running at your opponent. The Precision Strike is good, but IMO the Iron Zeal is what makes them better than the standard Ironfang UA.
Spriggan - this an amazing warjack for a number of reasons. First, it's the only non-character warjack (besides the Conquest) that has reach, plus the Powerful Charge ability as icing. Second, it has a shield. At first glance 1 more point of ARM may not seem like much, but the bump up numbers-wise from 20 to 21 is significant and makes the already resilient Khador chassis that much more so. Third are it's grenade lauchers. Though only RAT 4 like all Khador jacks and though it's only pow 10, having it be and AOE means you don't need to hit to be able to kill squishy enemy infantry like Mechanithralls, Kayazy or Nyss. The bette GL ability is Targeting Flare, allowing you to ignore stealth of models you place the AOE over. The fourth reason it's great is the Bulldoze ability, allowing you to push models be advances into base-to-base with up to 2" away. It's excellent in scenario to move enemy heavies out of zone/away from flags or objectives and can also be used to push models back to either: a, free up ranged units; b, push things away so you can charge them again; c, push enemy heavies that don't have reach away to free up your Spriggan to go attack something else.
The Behemoth - though pricey in points and $, this jack does it all. It's armor piercing fists can wreck nearly anything it touches that's on a medium or larger base and the two bombards ensure it gets plenty of use outside of melee as well (or in melee, as it is one of the few models that can actually shoot while engaged.)
There are a number of other things that are good either on their own or better with certain caster combinations, but the above can stand alone. To answer your original prompt, Karchev wants more warjacks (Behemoth, Spriggan and multiple Juggernauts), Strakov wants anything melee (Ironfangs, Doom Reavers, Kayazy, Kayazy Eliminators and the Great Bears) and Zerkova wants to bring stuff that's innately good to make up for the mediocre contributions she provides. Hope that helps. Feel free to ask more specific questions in the future.
Warmachine/Hordes Battle Reports
Those smote by Mulg the Ancient: Stryker2, Constance Blaize, Nemo2, Amon, Karchev, Skarre1, Venethrax, Magnus1, Madrak1, Jarl, Kruger1, Kruger2, Grayle, Morghoul1, Mordikar, Rhyas, Thagrosh1, Lylth1, Arkadius, Barnabas, Rask