If something is the runniest, it has the most liquid and is the least thick.
The runniest things are often wet and can make a mess.
Examples:
The runniest ice cream melts the fastest.
The runniest part of the soup spilled on the table.
The runniest part of the river is over there.
The runniest paint is the hardest to use.
The runniest water in the world is in that big waterfall.
The runniest mud is the messiest to play in.