The Bible doesn't specify any of the 10 Commandments as the most important. In the New Testament, Jesus says the greatest commandments are loving God completely and loving your neighbor as yourself, but neither of these are part of the original 10 Commandments. Judaism gives all the commandments equal importance but does denote some commandments as the most fundamental.
The 10 Commandments include not having any other gods, not making graven images, not using God's name in vain, keeping the Sabbath as a holy day and honoring one's parents. The remaining commandments are not bearing false witness, not coveting, and not committing murder, adultery or theft. The order of the commandments varies depending on the religion.
Opinions on the most important commandment vary for each person. For example, AchieveBalance says that the commandments are in order of importance, which is why not having any other gods comes first.
Each commandment is short and open to some interpretation, leading to debate on the actual meaning of them. For example, Albrecht Alt, a scholar of the Old Testament, wrote that the commandment prohibiting stealing actually prohibited abducting people or taking people as slaves. While there is a commandment prohibiting murder, it doesn't prohibit justified killings, such as in self-defense or war.