The answer is all of them. English is a Germanic language.
A far better question to ask would be "What other Germanic language is linguistically most like English?" (Hint, the answer would not be "German").
The problem with the original question is that it seems to imply that the language we today call "German" is the root of the English language, and thus there must exist some dialect of it which has diverged the least from "true German".
What instead happened was that some speakers of a (low) German language moved to England and slowly their language developed on its own, mostly isolated from the original tongue on the continent, into what we call "English". Meanwhile people on the continent had their own dialects which evolved into what we today call separate languages such as "German", "Dutch", "Danish", "Frisian", "Norwegian", etc.
All of those, and English too, are "Germanic" languages. Whatever exact language was spoken by the early German immigrants to England (let's call it "Germanic"), modern Germans would be no more able to understand it than they can English. If you don't believe me, pick up a copy of Beowulf (written in Old English) and see how easy it is to read.
Oh, and I understand the answer to the question I posed, is "Frisian". It is the sort of light-tan area in the map of Germanic language areas below.