Here are a few options, with pronunciation guides using a rough approximation of English sounds:
* Είστε καταπληκτικοί (Eiste katalpliktikoí): This is a formal but common translation. "Eiste" is pronounced roughly like "EE-steh" (the "steh" is a softer sound than "stay"). "Katalpliktikoí" is roughly "kah-tah-pleek-tee-kee" (the "i" at the end is a short "ee"). This translates to "You are amazing" (plural, formal).
* Είστε φανταστικοί (Eiste fantastikoi): This is a more informal option, meaning "You are fantastic." Pronunciation is similar to the above, with "fantastikoi" pronounced roughly as "fan-tah-stee-kee".
* Είσαστε υπέροχοι (Eisaste yperochí): This is a more literary and slightly less common option, meaning "You are wonderful" or "You are superb." "Eisaste" sounds like "ee-sah-steh". "Yperochí" is roughly "ee-peh-roh-hee".
The best option depends on the context. If you're talking to friends, the informal options are better. If speaking to elders or in a more formal setting, the first one is more appropriate.
Remember that Greek pronunciation involves many subtleties not easily captured in a written transliteration. Listening to a native speaker is the best way to learn the correct pronunciation. You can find audio pronunciations online using a Greek-English dictionary or a pronunciation website.