Which of them had the most impact and relevance?

Rihanna

Interesting

We're missing quite a few people. Donna, Mary...

Out of the people from the poll, Whitney and Janet.

Whitney
Janet
Donna
Diana
Aretha
Patti

I agree.

Whitney, Janet and Mariah from this list

Rita Ora

the original trinity

Whitney, Janet and Donna Summer.
Donna Summer is one of the prototypes for the pop diva, as well as being ahead of her time musically. She defined an entire genre and era of music.
Janet is the performer's performer. No other female artist in the past four decades has had the same kind of influence and effect on the stage as she has. Another one who was highly innovative musically.
Whitney is the singer's singer, by far the most influential vocalist of the past four decades and the first modern-era pop diva.

In terms of breaking barriers, they were all thriving in a white man's world.

Donna's only "rivals" in the disco arena were the Bee Gees. From 1975-1979, she scored four #1s, 10 Top 10 hits, and had three consecutive #1 albums.

Janet's chart longevity is unparalleled. A total of 7 Top 5 hits from one album and 18 Top 10 singles in a row, including a total of 10 #1s. Her style of R&B was edgy and innovative, but had enough commercial feel to cross over in a huge way, hence the multiplatinum album sales.

Whitney broke the Beatles and the Bee Gee's records for most consecutive #1 hits, scoring 7. First female artist to score a diamond album, best-selling debut album by an artist EVER (at the time), had sold 85 million albums by 1993.

Their success, especially Whitney's, is significant, because it showed just how marketable black women in music were, and that they could have the same kind of commercial clout as their white peers, male or female and it represented a shift in the industry from male dominance to a more female-oriented space. There have always been black female artists who have rivaled their male and white counterparts (ex. The Supremes rivaled The Beatles in chart dominance and success), but since the arrival and success of Whitney, it just went to another level.

That's why the answer to this could never be Beyonce, Rihanna or whoever. They're nowhere near as significant, nor have they contributed to music/the progression of artists (whether in sound, image, or opportunities) as much as they have been afforded by people such as Diana, Aretha, Donna, Whitney, Janet etc.

Rihanna WW

Whitney, Janet and Donna, in that order.

Whitney is by far the greatest female singer of the last 40 years as well as the most successful and impactful black female artist of the last 40 years. Her fusion of pop music with gospel roots made her the standard for singers following after her. Her presence on MTV as a black female also paved the way for other singers such as Janet to receive heavy rotation. Her performance of the SSB at the Superbowl is one of the most iconic moments in music history. It not only served as a template for future NA performers to follow, it presented Whitney as a symbol of hope and patriotism. A honor that had not been bestowed upon a black female singer in such a way before her. She became a national treasure in 1991. She was of the 80's and 90's what Aretha was of the 60's and 70's. A true diva.

Janet is the greatest female entertainer of the last 30 years, without a doubt. No other black female artist has pushed the visual medium forward like she has. Her storytelling through dance, music videos and live performance was not only innovative, but it paved the way for dozens of male and female artists. Musically, she has been at the forefront of Contemporary Pop and R&B music for over 30 years. Her Control album combined pop, R&b and funk with hip hop sensibility, thus inspiring a new genre known as New Jack Swing. Her evolving image and socially conscious and sexually liberating lyrics makes her one of the most heralded female artists of the last 40 years.

Donna, the Queen of Disco. She represents an entire era of music and was at the forefront of the Disco era. Her commercial and critical success with Disco inspired her own contemporaries to duplicate her sound. She was also ahead of her time musically with "I Feel Love." One of, if not the first electronic songs. Donna basically ushered in a new genre and innovated another. She's a jewel.

Whitney Houston.

Rihanna WW

Whitney and Mariah, wothout a doubt, especially WW (Rihanna is more global than them, but she lacks the talent or impact of the other two)

Out of all of those? I would say Janet which seems a bit weird since she's sold the least out of all the "legends" but she's just as huge a blueprint for the modern female pop star as Madonna. Britney especially.

In my opinion, Janet is the most impactful black female. The Control album pioneered the new jack swing genre, and provided the singer-dancer aesthetic of the video age. Every video artist of the MTV age has been influenced by Janet. Whitney is a close 2nd. Donna Summer is 3rd. Diana's peak was technically past 40 years ago so I won't include her.

Janet and Mariah, Whitney never really had musical impact on the same level of others listed. Sure you've had Xfactor rejects emulate her style but no one of worth.