Wargaming Super League

[Big Insect]

Nicophorian Byzantines tend to be a killer army in many sets of rules
It is probably to combination of combined arms - e.g. units with both a strong missile and also a melee capability in the same units - plus controllability and flexibility - combined with a lot of troop types to choose from. Other than elephants or camels (& war wagons) they have pretty much the full arsenal of troop types for a gamer to choose from.
Early T'ang Chinese (or Sui) are similar IMHO.

Alexandrian Macedonian is also a strong contender - it's that mix of high quality troops (Elites or Veterans) and the combo of impact Heavy Cavalry and the Phalanx (Hammer & Anvil), with good support troops.

I've always struggled with Mongols - I like the idea of them, and find they can work very well in smaller scales on bigger tables where there is room to manoeuvre and use their flexibility. But in 28mm games they usually end up back-off their own table edge by other larger cavalry armies.

In period, I agree about the Later Assyrian range of armies - again the combo of good quality infantry and hard hitting heavy chariots works well.

Having said all that, one of the top draw 'killer' armies in the ancient/dark-age world currently is the good old Africa Vandal - which it is possible to field as an army using a single troop type - hard-charging impetuous Heavy Cavalry (Sarmatian can be fielded in a similar manner). But these are pretty simplistic in their mode of operation e.g. line 'em up and charge!

Having said all that it is very dependant upon rules sets - if we were to take FoGR as an example - we'd all be speaking Tartar and the whole of europe &the middle east would be under the yoke of the Golden Horde ... 

[Lord Kermit of Birkenhead]

Always found Huns very effective, admitedly in 15. Never lost a game in FoG with em.