Which planet spins in the opposite direction to the others?

Venus. Nearly everything in the Solar System turns the same way, inherited from the rotation of the disc it all condensed out of - so a planet turning backwards needs explaining. Venus rotates clockwise seen from above the Sun's north pole, and does it so slowly that a Venusian day is longer than a Venusian year.

There are two competing accounts and neither is settled. One is a giant impact that flipped or reversed it early on. The other is subtler and now more favoured: Venus's thick atmosphere, which rotates far faster than the surface, exerts a tidal torque on the planet, and combined with the Sun's tides on the solid body this may have slowed the original spin to a stop and then pushed it backwards. On that reading the planet was not hit but talked round by its own air.

Uranus is the other rotational oddity, tipped over about 98 degrees so that it rolls rather than spins. Between them the two exceptions are why planetary scientists treat the tidy account of the Solar System forming as a smooth disc with some suspicion - the early system was evidently violent and messy enough to leave two planets that do not obey the pattern.