The Boomer as that's the one who starts it all; the massive confusion from which every other Infected can profit. It blinds survivors and forces them to stop for a while, defending against the horde of regular infected you just made.
It gives the Spitter an opportunity to split them up or just damage them outright when they're corner camping for the duration of the puke, it gives the Jockey an opening as the survivors are blinded, it gives the Smoker an opening for the same reason and as the survivors are pretty much forced to stand still for a while it makes the Charger's job a lot easier too. The Hunter can then pounce anyone who tries to rescue one of the others.
All it takes is one well-played, well-timed Boomer and it's all over. No other Special Infected has that ability, save for the Tank but I don't think that one counts.