Rina was my initial bias because she said she was a big studio ghibli fan in her first vlive and I liked her little princess vibe I feel she's a bit similar to Umji. She's also great dancer ofc and I like her vocal color. She's also brutally honest about her need for alone-time which I relate to.
As my Kpop years go by, I don't really have biases anymore, I tend to like a whole group. But anyway, in answer to you question, it was Sei who I noticed most to start with, then probably Suyeon, then Lua. But as I said, a sort-of like them all. Weki Meki come across to me like one single package.
Like a lot of Weki Meki fans, I felt sad for her Lua when she never seemed to get enough focus in their songs, but that has completely reversed in the last year.
As a bit of an aside, I remember getting into them partly because I wondered who were the girls staring open mouthed at a Taemin performance!