Venus has the slowest rotation rate. It takes Venus 243.7 Earth days to rotate once.
Planet Period
Venus 243.7 (Earth) days Mercury 58.8 days Pluto 6.4 days Mars 24.6 hours Earth 23.9 hours Uranus 17.2 hours Neptune 16.1 hours Saturn 10.7 hours Jupiter 9.9 hours Interesting . . . the largest planet rotates the fastest. I wonder why that is.