The most powerful units in the game are high tier archers (mounted or on foot). They are the decisive factor in combat. Therefore IMO the best T5 infantry in the game are those that best shield archers from enemy assault.
In this respect I'd suggest the actual best T5 infantry are probably Oathsworn or Wildlings - as they have a shield, a braceable polearm and a javelin - and according to the T5 vs T5 tests, are also OK in a melee (if not the best). I'd follow this with Sergeants, with their shield and braceable polearm.
But of course, realistically, discovering a 'best' unit is silly. Because the best 'unit' is always one that combines multiple unit types in a flexible way that can respond tactically to a wide range of situations - In this context, I'd suggest the best infantry company is Battanian - with good general purpose Oathsworn and Wildlings, great attacking Falxman and supported by clearly the best foot unit in the game: Fians.
In this context, Sturgians are also stronger than their individual units might suggest. Their T5 infantry can clearly take more arrow fire than any other units in game, as do their heavy cavalry - and like I said, archers are the decisive units in game - and thus they can close the gap more intact than other factions can - they might struggle in a melee, but they're more likely to get to a melee with greater numbers. Their archers might be less capable, but they also take less fire behind the game's best meat shield. All Sturgians really lack is a braceable polearm - leaving them vulnerable to heavy cavalry.
I tend to find that to defeat an infantry heavy Sturgian force, I have to resort to darting behind their shield wall to get them to turn their backs to my archers... otherwise I get tangled in melee which I don't want because I tend to play light on infantry.
Strengths and weaknesses can balance across companies, and all factions have strengths that can be utilised effectively by the player.
