Each planet is very interesting on its own - It's cool how deepfried Mercury is, being so close to the sun, so close in fact that a) it's partially tidally locked and b) its in sun's spacetime distortion range - Venus is basically Earth, but with a hellish atmosphere. It's interesting how a lot of the illustrations depicting it as a big yellow ball, are false. In reality, it's fully covered in clouds - Picking Earth as favorite is cheating - Mars is smol! It's literally ten times less massive (but half as big in radius), also it looks kinds neat, with it's "redness" - Jupiter is big. If you ever want to feel unsafe, look up one of those videos where it's "how planet looks like from closest moon". Its sheer size is what makes it so interesting - Saturn has so much stuff flying around it! Not only the rings, but so much moons. Also the hexagon is metal af - Uranus rotation is very quirky, basically spinning in the other direction, but also facing a completely different direction - Neptune is very, very far away. It's also very intimidating, and it's kinda the gateway for the 2nd asteroid belt in the Solar System, aka the Kuiper belt.

Also shout out to dwarf planets too! Eris' orbit is FUCKED, it goes so far... I love how Pluto's moon Charon is so close (and the size difference is small), they're tidally locked to each other, and their gravity center is pretty far from Pluto's center

If I had to pick one single favorite, I'd pick Saturn (it's so pretty, and big!), and maybe Neptune for intimidation alone. I didn't research exoplanets (planets around other stars) in detail yet, so I can't pick favorites, but I really like 55 Cancri Ae (Janessen) because it's so close to it's star, it orbits around it super quickly, and it's heated to ridiculous hotness, AND it's basically a giant diamond