Almost all the players I know owns a Roman army. I started collecting these barbarians long ago, mixing a lot of different figures: they are mostly Old Glory, Essex, Donnington and Chariot.
This army is perfect for a player willing some morphing. Can be adapted to many dark ages lists. I play it mainly as alamannic and gothic hosts.
I use it mostly in club campaign, and sometimes I've been unwise enough to play it in tournaments.
These are the generals, mounted and on foot. As KnF and WbS they enjoy quick kills results, but always attract unwanted attention by enemy.
The mainstay of most barbarian armies, mass of warbands
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Javelinmen, I regard them as elite troops
Heavy cavalry, an excellent complement to try to trample enemy lines. When KnF are fielded, the game is usually short.