I always thought that the smaller planets would rotate more rapidly than larger ones, under identical conditions of course. Much like a figure skater draws in his or her figure and seemingly rotates faster. However, I may be (and more than likely) am wrong. If someone could address this I would be interested to learn
Planet Speed at equator (km/h) Mercury 10.9 Venus 6.5 Earth 1670 Mars 867 Jupiter 45600 Saturn 37000 Uranus 10900 Neptune 8460 Pluto 47 You would expect small planets to spin faster - from conservation of angular momentum But larger planets were formed from larger clouds of stuff. As stuff contracted, then because of conservation of angular momentum it speeded up, the bigger/faster the original cloud the faster it ended up. Of course if you took the existing planets and made each of them smaller - they would speed up even more. Then there are effects that have happened since. Mercury's rotation is slowed by tidal friction with the sun so has a very slow speed (long day), the Earth's is slowed a little by friction with the moon. Uranus probably got hit by something in the past - which is why it has a weird axis tilt. Mars might also have been affected by whatever caused the asteroid belt.
A good way of comparing spin in stars is to compare angular momentum and mass in geometric units where- [tex]a=J/mc[/tex] [tex]M=Gm/c^2[/tex] where [tex]J=vmr\,k[/tex] where [itex]a[/itex] is the spin parameter in metres, v is the equatorial rotation velocity, m is mass, r is the equatorial radius and k is the moment of inertia coefficient (0.4 for an idealized sphere of uniform density). a/M produces a unitless figure between 0 and 1, the higher the number, the higher the spin. For the Sun (k=0.06), a/M=~0.188, for a 2.2 sol neutron star with a frequency of 1500 Hz (k=0.35), a/M=~0.488. This doesn't appear so straightforward with planets as M works out considerably smaller than [itex]a[/itex] but there should still be a way of comparing spin geometrically. EDIT: In the case of planets, you could probably get away with just considering the results of [itex]a[/itex] which is considered to be the amount of angular momentum per unit of mass (sometimes expressed as J/M). In this case, Jupiter is the clear winner and Mercury has the least 'spin' per unit of mass.