I have a few, none particularly more than the others.
I love the design (Magnezone should've been nine Magnemites, fuck you), I love how versatile it is in combat, and I love how, no matter what region I'm in, I can trust that a Magnemite will be nearby. So far it hasn't been confirmed for Galar, but I'm keeping my hopes up. If I'm not doing a Nuzlocke/Monotype/etc., I'll usually try to get a Magneton on my team.
Animated weaponry is one of my favorite monster types in non-Pokemon things, so I was excited when I saw Honedge. Doublade was a really fun tank (still is fun with Eviolite), and Aegislash is exactly what I'd want in a weapon Pokemon. The Stance Change lets me "fence" with my opponent. King's Shield + Shadow Sneak gives me a parry and riposte. I've had more fun in gameplay with Aegislash than just about anything else. And then the Pokken DLC hit, and Aegislash is fun enough to be one of my mains (Gardevoir and Shadow Mewtwo as the others).
First - the design. I like Pokemon that distinguish themselves from being just another technicolor animal, and we've got a Nazca line pottery bird. The concept of automaton sentries - fully biological - left behind by an ancient civilization is really interesting, and reinforces the unique design Earthbound Immortals notwithstanding. Then, the gameplay. Magic Guard is a great ability both to power through the campaign and to mess with people in competitive (a status Orb is a Safeguard on this bird), and Stored Power is one of my favorite moves (which also lead me to like Necrozma a lot, just not as much as Sigilyph).