The most common, and probobly best. White opens lines up and controls the center. According to chessgames.com, white has a 52.2% chance of winning, so it is not bad.
6.0-0:
Also respectable, but uncommon. It does not open up lines in the center. 51.9% chance of victory for white**.
6. Qb3:
Attacking f7, but not much else. Not bad, but not as common or as good as d4 or 0-0. In 19 games, white has a 47.4% chance of victory, but so does black.
** All percentages of victory and/or defeat are taken from chessgames.com.
6.d4:
The most common, and probobly best. White opens lines up and controls the center. According to chessgames.com, white has a 52.2% chance of winning, so it is not bad.
6.0-0:
Also respectable, but uncommon. It does not open up lines in the center. 51.9% chance of victory for white**.
6. Qb3:
Attacking f7, but not much else. Not bad, but not as common or as good as d4 or 0-0. In 19 games, white has a 47.4% chance of victory, but so does black.
** All percentages of victory and/or defeat are taken from chessgames.com.