EXO: the vocals and the music (I love that 90s-00s r&b). The fact they were fine as hell was just a plus. I liked their dance and performance too, but it was really the vocals.
NCT: the vocals and the music (again with the r&b plus I love hip hop - I love their swagger). I passed on them for a bit and then they brought me in - I always admired their dance and performance and now love it to bits - no one else is like them. First NCT U and NCT 127 (Taeil! Doyoung! jaehyun warming it up, Haechan adding the seasoning plus a pair of great rappers in Mark and Taeyong. I like Yuta and Jungwoo and Johnny too but the voices that brought me in were the mains), then WayV when they launched. It took me until Hot Sauce album to vibe with Dream. Also fine as hell, lbh.
Purple Kiss: the vocals and the music. Dance and performance a plus.
I grew to get and know and love the personalities of EXO and the NCT units but that's not what initially attracted me. I always fall for a voice (or voices). I can pick a voice out of a group before a face or a name.

For Purki, it really is about the vocals and music and performance and I don't know as much about them as people.