tier has 3 distinct synonyms spread across 2 of its 5 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a relative position or degree of value in a graded group
“lumber of the highest grade”
any one of two or more competitors who tie one another
something that is used for tying
“the sail is fastened to the yard with tiers”
one of two or more layers one atop another
“tier upon tier of huge casks”
Synonyms, antonyms and the broader and narrower relations for tier come from Princeton University’s WordNet. Full sources and licences.