Drops with paying attention - Banelings. Typically some attention is required however as many players will start to move workers away and you need to micro to ensure they don't blow up on production buildings.
Drop and forget - 8 Lings I find to be more effective than 4 roaches, so I'd go for lings with a bonus of cost effectiveness.
Money no object - 4 Hydra's, they are actually the best for DPS and they tear through buildings doing a lot of damage. Just comes down to the fact you will most likely lose them and it's 400 / 200 being traded.
I agree with the first one but the others are terrible. Chances are that vs a toss he will have observers that will spot it, zerg will have overlords to spot it and terran will have units in their main to kill and spot. 1 overlord full of units is not worth it at all. Toss will warp in 2 zealots, zerg will send back a few units and terran will pew pew the shit out of the overlord. Same with dropping hydras, the only profitable way of doing that is multitasking the shit out of your opponent, dropping banelings on 2 mines and hydra drops at the same time, as well as pushing out with your main army ( this is super hard to do)..
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