All the factions are pretty much tied, for every boon they have they usually have a debilitating chutch.
Imperium, largest empire, with the largest armies, could easily crush any foe but is too supremely ineffiecent to get much of anything done. By the time they finally react to something the damage is already done.
Chaos can never truly be beaten but it fights itself as much as the enemy.
Eldar, powerful but few and scattered. Only reason they havent been wiped out yet is because they're home worlds are on the move and are thus hard to attack directly.
Necrons, extremly powerful, but suffer from any number of problems, such as they're programing failing. to them all potenially suffering a virus that turns them into crazed flesh eating monsters.
Tau, unmatached at ranged, but their morale is shattered when an etheral dies.