Depends on what you're doing. Dumps of multitrack CD media should always be bin+cue or a compressed version thereof, such as chd. DVDs and Blu-rays can dump as iso. There are also some extremely niche cases such as specific copy protection that require mdf+mds for a proper dump, but that won't be something the average user ever encounters. Basically, those formats exist and are still used for a reason, whether you understand them or not.

I do reserve some hatred for people who dump PS1 games as iso, or who use ccd+img+sub for things where the subchannels have no valid usage.