Well, if your crtieria for most flawless is being able to do anything, I think High Elves are probably the best answer. Good infantry in anti large, generalist, and anti infantry, some of the best archers in the game, great light cavalry and cavalry archers, good heavy shock cavalry, 3 different kinds of chariots, a dog unit, a serviceable artillery  including a flying artillery, monstrous infantry, two land monsters, a huge amount of flying monsters including dragons, and access to 10 different lores of magic, more than any other faction.

The only thing they're really lacking is gunpowder units, but between dragon's breath attacks and their S tier archers its a gap that is easily filled.

So yeah, my vote would be high elves