Possibly 'thickest' but dense has a particular meaning and does not lend itself to being substituted with synonyms.
Other meanings of the word could include the following ~
Synonym.com also suggest impenetrable, heavy, dull, dumb, obtuse slow.
Which planet is the densest in our Solar system? The densest gas will drop to the bottom of the glass. Is the Amazon jungle the densest in the world?
The densest elements are osmium and irdium.
Earth is the densest planet.
The densest part of earth is the core.
The densest stars are known as black holes.
Northwestern Europe has the densest population.
The densest material settles to the core.
The most densest planets are Earth and Mercury
The densest layer of the earth is the inner core.
salt water is densest
Strong (i.e bread grade) wholemeal flour is densest.
The two densest physical layers of the earth are the lithosphere and the asthenosphere.