Eloquent, fluent, articulate, expressive are adjectives that characterize speech or speakers notable for their effectiveness.
Eloquent suggests clarity and power:
an eloquent plea for disarmament. Fluent, with a root sense of flowing, refers to easy, smooth, facile speech:
fluent in three languages. Articulate characterizes a clear and effective speaker or speech:
an articulate spokesman for tax reform. Expressive focuses on rendering intelligible or meaningful the ideas or feelings of a speaker or writer and implies an especially effective, vivid use of language:
a deeply moving, powerfully expressive evocation of a city childhood. See also
fluent.