What is the most deadly liquid?

Answer 1

Lava (or any molten rock or metal). But strong acids or bases can be deadly (e.g. hydrochloric acid, ammonia) as can various poisons and venoms.

But the one that kills the most people annually is water.

Answer 2

The most deadly liquid is generally considered to be dimethylmercury, a highly toxic and rapidly fatal substance even in very small amounts. It can be absorbed through the skin and is known to cause severe neurological damage.