Madonna - Her work ethic for over 30 years is unparalleled. Especially from 1998 and onwards with a constant state of album and tour alongside of directing movies, dabbling in new business/merchandising ventures and charity work. Whenever she does something major people will be talking about it good or bad. Goes without saying that even in 2016 she can have a lowkey year in the public eye but everything else she does behind the scenes can still get the Billboard Woman of The Year Award
Beyonce and Lady Gaga - Both do overtime making sure that Billboard and Rolling Stone are keeping a watch on their moves in music. Play an all stadium tour or play a dive bar tour and they'll want people to know. Not all the girls get that constant praise. Their music videos are also a mark in their hard work. Beyonce's were all pretty basic aside from Single Ladies and Run The World (Girls) until 2012 where that's where she puts a bulk of her effort in. Lady Gaga's right from the get go were made to make you look at her with outrageous outfits or high end fashion. Further on they were also like Michael Jackson in how he loved to present them as mini movies.
Taylor Swift and Katy Perry. They take their time to write all their songs, learn multiple instruments and the gp seems to agree based on their success.
Rihanna, Britney Spears and Adele are also hard-workers.