不可否认【bùkě fǒurèn】

不可否认 grants a point the reader might raise. It opens the sentence, followed by a comma, and is often followed in turn by 但是 or 然而 and the writer's real argument. The register is essays, reports and debate.

不可否认的是 works the same way, with a little more weight.

As a predicate or modifier

是不可否认的 closes a sentence: "is undeniable". 不可否认的 + noun describes a fact or a truth.

No subject

不可否认 is impersonal and stands without a subject. With 我 as the subject, use 我不否认 or 我承认.

无可否认 means the same and is slightly more formal.