My name might give it away, but my favorite word has always been えいえん. Back when I was still learning how to write the characters, it just had a nice, flowing feel to it and stuck with me.
It never really occurred to me to have a favorite, so I'm not sure I have one.
But I made a list of some words I enjoy:
闇雲: I feel it really evocatively conveys the sense of flailing around in the dark. Along similar lines, むやみに is also kind of fun.
羽織: The word itself is picturesque and evocative (a woven thing that's like wings that you wear--not sure if that's the real etymology, just what the kanji make it sound like), and I'm also sort of fascinated recently by 羽織 themselves. I learned that, like "coat," a 羽織 of any length is still a 羽織, but there are lots of different ways you can be more specific, like 長羽織、中羽織、茶羽織、綿入れ羽織、紋付き羽織、絵羽織、and so on. Source: https://kimono-story.com/entry...
逆鱗: Learned this from Pokemon. ;) The etymology is really fun if you read the J-J definition (spoiler: it comes from the idea of the outrage of a dragon if you touch a certain scale that is growing backwards).
巴: I was familiar with the name Tomoe but didn't learn until recently that 巴 is that comma-design you see everywhere. Like 羽織 there are different words for different types, in this case based on how many comma shapes there are.
斑: Not so much the word itself (though the kanji looks kind of cool) as that it's fun to recognize the humor when there are characters in Japanese popular media with this name, who like to act big, bad, and dangerous, but whose name/nickname actually means "Spot."
カモシカ：More the actual animal, but how カモ + シカ equals "serow" is really amusing if you think of it as 鴨 (duck) + 鹿 (deer) instead of the actual meaning （氈鹿、羚羊）.
EDIT: Actually, the real etymology of the kanji used to write カモシカ, rug-deer (氈鹿) (a deer-like creature that's shaggy like a rug?) or antelope-sheep (羚羊) (fleet and good at jumping, like an antelope, but shaggy like a sheep?) is pretty fun too!
My favorite word is えんぴつ, I don't know what it is about that word that makes my brain go "Yes, this is a very good and important word. Never forget it", but whatever it is, えんぴつ has it.
Don't know if it's exactly my favorite word, but 鬱 「うす」is probably one of the most memorable words I have since it was the first Kanji/word our Sensei taught us when we began kanji in 101 but then when explaining my absence in 201 due to a bout of depression the word was lost on her. Quite a memorable experience
