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.
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