Julius Caesar was important because he ended the republic, and he ended the democracy. He also conquered Gaul, and was the first emperor.
Augustus was important because he ended 100 years of civil war, along with that he achieved over 40 years of peace, and he was very clever with his decisions. He also was just 19 when he first Caesar died so he took rule, and for some of the decisions he made at such young of an age I Rome probably wouldn't be as well known as they are today.
Vespasian brought much wealth, stability, and built the coliseum. He also restored Rome after other rulers that would splurged their money.
Trajan expanded Rome, and gained much wealth for Rome. He also started a welfare program for the poor.