* Σ' αγαπώ (S' agapō): This is the most common and generally accepted translation. It's a strong declaration of love, suitable for romantic relationships. The "Σ'" is a contraction of "σε" (se), meaning "you."
* Σε αγαπώ (Se agapō): This is the same as above, but written without the contraction.
* Αγαπώ σε (Agapō se): This is grammatically correct, just a different word order. The meaning is identical.
Therefore, Σ' αγαπώ or Σε αγαπώ are the best choices for most situations.