It varies, sometimes its singers or dancers, when I first got into kpop it was rappers not sure why, it's very individual.
Hence why a bias usually jumps out to me around the 2nd or third comeback as it gives me enough listening material discography wise.
But I'm more vocal heavy I suppose.
Like for NCT.. I started with yuta and Ten (dance based) then switched to Haechan and do young, then dropped yuta added Ten back... And I'm looking at xiao Jun....yang yang seems to have a pull performance wise So... But that's generally a mess cause so many members like sungchan came in rapping I was like oooo... I like his tone...
For SHINee, its always been Onew but Its kinda a package deal like it hard for me to like one without the others...the lead Onew has for me a quite miniscule, but there there only group I really Stan.
I find it easier to stay with a female bias though from day one. RV it was Seulgi striaght from happiness, GF it was sinB from glass bead, still varies between dancers and singers though. I think the only female idol rappers I bias in groups are Miryo, bora and Moonbyul....Well as far as I remember.