To determine which planet takes the longest to complete one revolution around the Sun, we can look at the "Length of Year (Earth years)" for each planet. According to the table:
- Venus: 0.62 Earth years
- Earth: 1.0 Earth year
- Mars: 1.9 Earth years
- Jupiter: 11.9 Earth years
- Saturn: 29.4 Earth years
- Neptune: 164.8 Earth years
Out of these, Neptune has the longest length of year at 164.8 Earth years.
So, the answer is Neptune.