It depends on what the song needs! Some songs or parts of songs need a simple repetitive bass line that defines the song. Others need sophisticated lines to make the song sound good. Never play fast sophisticated lines just to show off how good you are (bass solos excepted) - it rarely compliments the song.This is probably genre and your skill dependent question, but when you compose your bass lines are you trying to keep them simple with some fills here and there or you want it to be as sophisticated and technically challenging as possible?
What your band mates expecting of your lines if you are in a band?
Listen to some of the stuff that Lee Sklar has recorded over the years or watch him play at those award shows that often have him playing bass. He plays what the song needs and puts in tasteful fills in all the right places at all the right times. Some of them are slow and some of them are fast and complex. He's genius at it and he makes it all sound good.