I really love the word 最近（さいきん）which means recently. I just really like how it sounds. Any words with the あい sound in them I like, but also I really enjoy writing out the kana for this one. The flicks on さ and き are so satisfying.
As for favorite kanji, at the moment it’s 茶. I’m a big tea lover so I’m pretty bias, but the character is just so elegant and nice to look at.
I’ve never really thought about having a favorite grammar thing. I guess I’d say verb normalization, since the first time I learned about it, it blew my mind (shameful promotion of my thread on mind blows in Japanese).
Also another word I just thought of that made me laugh the first time I heard it: 言語道断（ごんごどうだん）, meaning ridiculous / outrageous. Personally I think this is the perfect autological word (a word which describes itself).
Bonus words: 学校（がっこう）、花火（はなび）、おはよ、ありがとう、回る（まわる）、雪（ゆき）、する