It's hard to rate the second most (this may be the third most, for example). In the same account (Genesis 3), we see another common trick.
"God doesn't want you to enjoy life."We see this in Genesis 3:5 "For God doth know that in the day ye eat thereof, then your eyes shall be opened, and ye shall be as gods, knowing good and evil."
See! That mean, old God, He doesn't want you to have good stuff!
Of course, Satan is again with the half-truths and double talk. Adam and Eve already knew good and evil. Good is to do what God has said. Evil is to not do it. Eating from the tree is evil.
It's interesting that the Hebrew word for "know" (yada) is also used for the relations between a husband and wife ("And Adam knew Eve his wife; and she conceived, and bare Cain").
After eating from the tree, Adam and Eve (and all humanity) come to "know" evil very intimately indeed.
The truth is that God knows good and evil. He knows what will harm us, and what is for the best. To follow God's law is not salvation, but it is a good idea. It's not keeping you from "the good life", it's keeping you from evil.