Tenno are known for depopulating entire galleons before anyone has enough time to figure out what's going on. But assuming they're in open, active combat? Nidus.
The infested alone are bad enough seeing as how they can directly infect flesh and metal, and tear apart ships given enough time. An infestation controlled by a Tenno is definitely worse, being able to direct it wherever he may want it, and spawning his own little infested flora and weird bunny things that bore into you then explode. Then suddenly he throws a ball of tendrils which grabs your friends without any hope to escape, easy pickings. At the same time, you and any few other survivors of the initial attack are probably now infected.
With this being said I have to wonder... Ignoring any gameplay limitations, wouldn't Nidus' infestation just spread and mutate upon any ability usage? His first and fourth especially spawn masses of infested flesh and flora. Wouldn't any attacks orchestrated by him just cause a spike in infested outbreaks, albeit under the control of a Tenno?