This is a complex question requiring a complex answer, because it all depends on what kind of language you examine. Corpus evidence used in the ‘Longman Grammar of Spoken and Written English’ shows the following approximate frequencies of thousands of words per million:
LEXICAL WORDS
CONVERSATION
Adverbs 50
Adjectives 25
Verbs 125
Nouns 150
ACADEMIC PROSE
Adverbs 30
Adjectives 100
Verbs 100
Nouns 300
FUNCTION WORDS
CONVERSATION
Pronouns 165
Primary auxiliary verbs 85
Prepositions 55
Determiners 45
Coordinators 30
Modals 20
Subordinators 15
Adverbial particles 10
ACADEMIC PROSE
Pronouns 40
Primary auxiliary verbs 65
Prepositions 150
Determiners 100
Coordinators 40
Modals 15
Subordinators 10
Adverbial particles 5
These figures can give only a crude picture and show only the figures for one kind of written English. In general, though, nouns and verbs are the most common words, and conversation seems to use a higher proportion of verbs, adverbs and pronouns, while written English uses a higher proportion of nouns and adjectives.