i never heard of mutas and dark swarm before, it's the dumbest thing i ever heard. maybe u're trying to say mutas and plague.
i think dark archons can take out BCs, 250 energy, mind control one, then feed back 2 more. so 2 archons can take out like 4-5 BCs, mind control 2, feed back the other 3 and that will really hurt.
there are no most powerful units in sc, every unit have a flaw. that's why noone use only one unit except newbs. one of BCs' strength is it's canon, but that is it's weakness vs DA.
even if it's one vs one, there is no most powerful unit. since DA can mind control, but it can't do anything vs DTs, it can run and wait for ghost to run out of energy though, depend on map. DTs are invisible but it can't attack air, i'm not sure if irradiate work on DTs or not, i think so though, since broodlings work on em. queen can also kill dts with the green stuff, then spawn broodlings.