Finding decent Lupin translations can be a headache because the franchise is so scattered across time and mediums. The early novels by Maurice Leblanc are public domain, so you'll find a dozen versions on Project Gutenberg, but quality varies wildly—some are charmingly archaic, others are just clunky. For the manga and anime adaptations, official publishers are your safest route. I'd lean toward the 'Lupin III' manga releases from Seven Seas; they're consistent. Avoid aggregator sites for these, the scans are often blurry and the typesetting gives me a migraine. Online communities like the Lupin subreddit sometimes have leads on fan translations of the really obscure stuff, like the later novels that never got official English print runs, but it's a real mixed bag.
Honestly, the 'best' translation depends on what you're after. If you want the original Arsène Lupin gentleman-thief stories, the public domain translations by Alexander Teixeira de Mattos are a classic starting point, even with their period quirks. For the modern, zany 'Lupin III' vibe, the official anime subs and manga are professionally done. I wasted an afternoon on a site that claimed to have the 'definitive' novel translations, only to find they'd just run an old text through Google Translate. The dialogue came out like a robot having a stroke.