Venus has the slowest rotation on it's axis but Mercury has the longest day. This is because the apparent motion of the Sun in the sky of any planet is defined by the planets spin (rotation with respect to the stars) and it's orbit around the Sun.
Mercury's spin and orbit mean that the Sun only appears to go round the planet once every 176 Earth days from the perspective of someone on the planet surface. For Venus it's only once every 116 Earth days.
Click for a full explanation of why Planet Mercury has the Longest Day.