What is the best translation of the aeneid or what translation of the aeneid should you buy?

As with all questions of translations it depends on what you're looking for. For those who are looking to read for pleasure and want to keep things simple yet vivid, then Fitzerald, Fagles, and Lombardo are your best picks. None of them are extremely accurate to Virgil's original in terms of rhythm, syntax, diction, or Wordplay, but are highly readable and accessible in English. Of that group I would recommend Fitzerald. Moving upwards in difficulty/accuracy we have Allen Mandlebaum, whom I think strikes a good balance between extremes while remaining poetic. Mandlebaum is often my first pick for translations. F. Jackson Knight renders the work in prose, which means he's likely the most accurate Word-for-Word available, but you do lose the "poetry".

My current two favorite renderings of The Aeneid are Sarah Ruden and Frederick Ahl, with a preference for the latter. Ahl is a lifelong fan of The Aeneid and spent over a decade translating it, with the intent on staying as faithful to Virgil as possible on as many levels as possible. His copious notes also help illuminate the historical context and many of Virgil's references. There's also the classic John Dryden translation, but it was written in the 18th Century and may seem anachronistic today, but it's also some of the finest poetry ever written in the English language.