Depends on what level you're talking about, but overall, the Sicilian Najdorf (1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 a6). The databases are your friend here. However, at the very beginner level and the World Championship level, the Ruy Lopez (1. e4 e5 2. Nf3 Nc6 3. Bb5) is most popular.
The literal answer is the King's Pawn Opening (1. e4) but you probably meant openings going deeper than one move.
Answer it yourself