Which one of the Ten Commandments is hard to obey?

When asked "What was the greatest commandment," Jesus summarized all ten in two parts:
1. Love the Lord your God with all your heart and with all your soul and with all your mind.
2. Love your neighbor as yourself
(Matthew 22:34-40)
Jesus never singled out one commandment as being harder or more important - He placed emphasis upon the overall objective of the commandments in these two statements.

The Ten Commandments were meant to be a set of principles governing both worship and general ethics. However, they also directly address many flaws that humans lean towards depending on their upbringing and lot in life. Each commandment thus presents its own difficulties in terms of following it.
In the present day, Christians face temptations and opportunities to break nearly every commandment in many situations. For example, taking the Lord's name in vain warned against using God's name in oaths and interjections. In current speech, it's quite common to say "I swear to God" even if the meaning is facetious. How often are you tempted to lie about someone else? That would be bearing false witness against your neighbor. Stealing could be as simple as taking a stray grape from a grocery store produce display since it isn't your grape and you haven't paid for it. The commandment not to have any other gods means not to put anything above God in your life. Money, clothes, music groups, and even celebrities could all end up as "gods" if you spend more time and energy on them than God.
The difficulty in following the Ten Commandments largely varies from person to person and may also depend on the culture and historical period. The hardest one to follow for a given individual is likely the one that they do without even thinking about it.

In the teachings of the Bible, every one of God's Ten Commandments is supposed to be intrinsically hard for humans to obey. They stand in direct, equal opposition to mankind's primary flaws, which date back to the first two humans, Adam and Eve. The couple was expelled from the Garden of Eden after committing the first act of disobedience against God: eating from the Tree of Knowledge of Good and Evil. In committing the act, Adam and Eve gave all humans thereafter the desire to disobey God.
In real life (outside the Biblical abstraction), any of the Ten Commandments might be harder to obey than the others. Acts of deviance from defined rules, whether religious in nature or otherwise, vary according to the personality, history, and inclinations of each individual.