I'm kind of mixed as to which planet is my favorite - mainly since I like almost all of them: Here's my reasons:
Mercury: Granted, it's not too special, but, hey, it's pretty strong, considerig that it's only a tiny rock (figuratively) floating around so gosh darn close to the sun.
Venus: A very cool planet indeed (not temperature wise, of course! :p) The thing I find the most interesting is that it has a full atmosphere AND surface rock, and the only other planet to have such, is our great Earth. Too bad that Venus' atmosphere is comprised of several acids and other poisonous chemicals, not to mention the extreme surface temperatures.
Mars: The cool thing about this planet is that its the first other planet where life was rumored to exist. Granted, that hasn't been completely proven yet, but it's still interesting. Another thing about Mars is that H2O may have existed in liquid form at one point. Plus, the eerie pink atmosphere and oxidized iron rocks (rust) just add that touch of mystery...
Jupiter: Its HUGE. The pallete of reddish bands of color is second to none, and the ferocity of the storms which occur so often, not to mention the Great Red Spot, a huge storm that's existed ever since Humans first saw Jupiter. Plus, it's Jupiter that saves us time and again from incoming meteors and such, since its extremely high gravity level pulls metors out of thier path, acting as a protectve shield. Belive this - Jupiter actually does have a ring. Its very thin and hard to see though. But I have seen it myself.
Saturn: Saturn is cool, not only because its another planet made ENTIRELY of gases (although I heard somewhere that the core may be solid, but I don't know if that's entirely true) The ring is really cool too, especially since its comprised of bands of different rock types, supposedly moons that were too close, and got practically obliterated. They're held in place by the gravity which forces it to the equator of the planet. Also, the ring has a defined inner edge, since its at that point which the full effect of gravity pulls the rocks down to the planet surface. I wonder what the ring looks like from just above the outermost clouds at the equator...
Uranus: I like this planet, mainly because its the total oddball of the Solar System, considering how its axis is on its side, its completely freaky. Proof of that fact is the ring - it's vertically oriented.
Neptune: Not exactly terribly too interesting, but I really like its blue color, and it gives it a 'cold' feeling, aided by the fact that its so far away from the Sun.
Pluto: Aah, Pluto. And Chiton (sp?) They revolve around one another, they're the same size, and they're so darned far out. The Sun looks like a really bright star from that distance, and it has no atmosphere whatsoever. Most of it is actually comprised of some sort of Ice, as I've heard. But I do know that it has a solid core - of I think Iron. I just wonder how the gravity is affected by the pairing of its moon, since they revolve AROUND EACH OTHER on an axis somewhere in the middle. I doubt it really makes a difference though, the gravity is so low anyway, that you could probably escape it with a single good jump.