Everyone saying Britney and Rihanna when this thread was obviously made by the OP thinking that everyone would break a heel running in here just to say Katy Perry.
Britney Spears
Katy Perry
Ke$ha
Taylor Swift
and Mariah Carey nowadays
Don't be so ignorant. I'm not a fan of Katy but at least she knows how to hold a guitar. Taylor has a full acoustic set on tour, just her and her guitar or piano. Even Britney can play the piano and sing at the same time if she tries, lol poor you if your fave can't do it, because even Britney is capable of that.
Britney then JLo/Avril and it's not even debatable. You can hate Katy, Rih, Gaga, Madonna, Bey, etc... But they are not even eligible for "worst performers" when there are popstars like the 3 mentioned first.
I can't believe people are actually saying Katy Perry. The OP said "without big production, a fancy stage and dancers." All these things make Katy's vocals worse. She can't sing and dance/move as well as her contemporaries like Gaga and Beyonce who do it brilliantly and effortlessly. It's mostly because part of Katy Perry's vocals involve switching her key frequently. If you've ever seen Ariana Grande impersonate Katy Perry, that's what she impersonates - her key switching.
If you take away Katy's stage, dancers and big performance, her vocals would actually be so much better because she wouldn't be so breathy and wouldn't miss the notes she has to hit when key-switching because she wouldn't be running around/dancing.
Some examples...
She's best when she's standing still. Getting rid of all the dancers, dancing, lighting, movement, running around the stage and big fancy stage would only make her better.
She's a pop girl now. Damn she surely upgraded from Hip-Hop to Pop fast.
Well, yes. Are we going to call Taylor a country girl because the majority of her discography is country? Or Gaga an electronic girl because she's mostly electronic? Or Beyonce and Urban girl because she's mostly urban? She's pop because she's popular. The majority of people know who she is now because she has a #1 song and has had so many live TV performances now. Not to mention the hook/chorus on Fancy is definitely pop.
Besides, you're splitting hairs. That's not what's important here. It's about who would have the most difficulty performing if it was just them and their voice on stage and I think it would be Iggy because I think her vocals are terrible. And she's not even singing...there's no notes to miss and it's not possible to be off key. She just naturally doesn't have a good voice for rap.
Damn she surely upgraded from Hip-Hop to Pop fast.