In my ~*perfect*~ ARTPOP era, there are like 5 choices for 6th single tbh: “MANiCURE”, “Jewels n’ Drugs”, “Fashion!”, “G.U.Y.”, and “Dope”.
"MANiCURE" would be the prime candidate. It’s rocky-ish sound might be a little different for the GP, but everything else screams smash tbh. I could actually see this becoming a top 5 hit, and, depending on the video, one of Gaga’s more memorable singles.
"G.U.Y." is a little iffy with me. Almost every stan seems to think this could be gaga’s next "Bad Romance"/"Poker Face" commercially, but I feel it might just be a little too "different" to make a big dent in the charts. The beats are very abrasive, and the concept of the song would either be considered too explicit ("SEX IS SO UNNATURAL BANISH IT SAVE THE CHILDREN") or, a description used to Gaga’s describe career since it started nearly 6 years ago, too weird ("lol wtf is she talking about girl under you?? grave unearth you?? g.i.r.l.??"). but who knows, it might just be the most well-executed combination of "weird" and "sexy" Gaga has done since "Bad Romance", so it could strike a chord with the GP. Anyways, it’s my 2nd choice for 6th single.
"Fashion!" and "Dope" are tied for 3rd choice. "Fashion!" has been descibed as a filler on ARTPOP, but I feel it’s fun and cute enough to be a hit. It would be up to the public to decide whether or not it’s disposable. “Dope“‘s vocals may be a little too raw/slurred for casual listeners, but Gaga’s last ballad (“You and I”) performed decently, so if it has a big performance at the Grammys/VMAs, it could do some damage. And even if it doesn't, it still has that #8 peak.
"Jewels n’ Drugs" is my last choice for 6th single. It may be agreed upon as the "worst" song on ARTPOP but I feel this has some potential. It also could benefit (if lucky) from radio play assistance from rhythmic/urban radio.