Originally Answered:

How do you pronounce the vowel ē in Ancient Greek?

Contra Joonas Vakkilainen: η is likelier to have been /ɛː/; /eː/ is what ει was pronounced like in Attic, either as a monophthongisation of Homeric ει /ei/, or as the development of Homeric εε /ee/. In which case no great matches in English, but Received Pronunciation there (without the r) is close: International Phonetic Alphabet chart for English dialects.