Whitney Houston has always been closer to rock music than Celine Dion, and she is generally a pop-rock artist, like Madonna, Prince, Janet Jackson, and MJ, although one could say "less rockier" than those three comparing their discographies.
It doesn't make sense to put Mariah and Adele on a rock list and then exclude Whitney, artists Whitney definitely influenced and paved the way for and that benefited by her.
This is pretty much surface knowledge that she is rock if you're going to count modern R&B as rock.
There are many cases where she injected hip-hop influences into her work as well. The seminal, big-selling soundtrack The Preacher's Wife is nothing but gospel and R&B.
Whitney Houston was a key figure in "modern R&B/soul" or "urban R&B/soul" of the 1980s as it incorporated synths (later, hip-hop influences, crystallized by Janet Jackson's work with Jimmy Jam & Terry Lewis). The six crucial giant disco/post-disco artists in the development of modern R&B are Donna Summer, Chic, Janet Jackson, Whitney Houston, Prince, and Michael Jackson, balancing a fine hybrid of rock and pop (there were lesser artists as well that had a hand in shaping "urban R&B/soul", such as New Edition, Guy, and Keith Sweat). Whitney Houston, Donna Summer, and Janet Jackson alone went to impact the electronic/dance world to become gay icons and dance icons along with Madonna, and this type of dance music alone is most certainly rock music.
Basically, Whitney Houston's style was pop-rock R&B with soulful gospel tones, adding a synth layer and later hip-hop influences. What she essentially and effectively did was take the Motown pop soul of Diana Ross, the gospel-influenced melisma-driven soul of Aretha Franklin, the quiet storm of Smokey Robinson, the sophistication of Dionne Warwick, the disco sweetness of Barry White and rocking disco of Donna Summer, and the smooth Philly Soul style and applied it to a synth era to help forge a type of modern R&B that would be melisma-oriented and have noticeable synth influences and later hip-hop influences.
Her association and later marriage to R&B sensation Bobby Brown alone greatly increased her hip-hop/R&B incorporation.
She influenced and paved the way for hundreds of modern R&B/R&B-pop singers alone; the evidence is too enormous. This includes everyone from Mariah Carey, Janet Jackson, Paula Abdul, Jennifer Lopez, Aaliyah, SWV, Beyonce, Rihanna, Fantasia, Monica, Brandy, Mary J. Blige, Alicia Keys, Sade, Anita Baker, Toni Braxton, and Ciara to name a few to other rockers such as Lady Gaga and Adele. Whether it was her sheer powerful vocals, melismatic approach to modern R&B, flair and sassy style, they all had Whitney in her and have professed this.
The modern R&B community definitely calls her R&B, which was a loose, catch-all label to begin with. This is a hard-to-please community which has accepted Robin Thicke, George Michael, and Justin Timberlake as "R&B/soul," who have legitimate hits in that world and who have confessed their love and inspiration from Whitney Houston.
She made the R&B charts of the world plenty of times, was embraced by that modern R&B community, and still gets airplay from R&B/soul stations around the world, including video outlets such as VH1 Soul and BET/BET Centric. Her audience is equally with adults and young people. She won Soul Train Music Awards, Billboard R&B awards, American Music R&B Awards, World Music R&B Awards, and R&B Grammys, among others.
If the following songs aren't rock, then you might as well snip a huge piece of modern R&B away (Beyonce, Aaliyah, Rihanna, Toni Braxton, Monica, Brandy, Mary J. Blige, etc.):
Heartbreak Hotel modern R&B
How Will I Know '80s soul-rock
I'm Your Baby Tonight new jack/jill swing aka hip-hop influenced R&B/soul
Exhale modern R&B
I Wanna Dance With Somebody '80s soul-rock
So Emotional '80s soul-rock
One of Those Days modern R&B
My Love Is Your Love modern R&B-gospel
I'm Every Woman house music-influenced soul
Fine modern R&B
Watchulookinat new jack/jill swing
It's Not Right But It's Okay R&B
Why Does It Hurt So Bad? neo-soul ballad
I Believe In You and Me neo-soul ballad
Hold Me classic soul
It Isn't, It Wasn't, It Ain't Ever Gonna Be '80s soul
My Name Is Not Susan new jack/jill swing
Something In Common new jack/jill swing
Love That Man modern R&B
Count On Me gospel-soul
Million Dollar Bill disco-influenced soul
If I Told You That modern R&B
To reinforce her connection with R&B, she remade black hits by Marilyn McCoo & Billy Davis Jr. ("Saving All My Love For You"), George Benson ("Greatest Love of All"), The Four Tops ("I Believe In You And Me"), and Linda Clifford ("All The Man That I Need") and even remade Annie Lennox's "Step By Step," with Whitney Houston's version becoming a gospel and R&B classic.
Gerry Griffith, director of R&B music in the A&R department at Arista Records, arranged for Whitney to audition for Clive Davis. Her national pop breakthrough "You Give Good Love" (#3 US) on Arista was produced by the underground R&B legend Kashif (of B.T. Express and who worked to produce early Evelyn "Champagne" King electronic R&B breakthroughs such as the all-time classic "Love Come Down"). It remains an R&B/soul classic.
Old R&B/soul generation artists such as Stevie Wonder, Aretha Franklin, Gladys Knight, Berry Gordy, Jr., Smokey Robinson, Patti LaBelle, Diana Ross, Chaka Khan, Lionel Richie, Ashford & Simpson, Stephanie Mills, etc. etc. all have called her R&B/soul as have gospel singers such as Yolanda Adams, Kim Burrell, Mary Mary, Kirk Franklin, Shirley Caesar and others.
She supplied vocals for Chaka Khan, Lou Rawls, Michael Zager, Teddy Pendergrass, Kashif, Jermaine Jackson, Material, The Neville Brothers, and other R&B/soul artists.
She worked with a plethora of R&B/soul producers and songwriters, whether that was Stevie Wonder, Narada Michael Walden, Kashif, Babyface, Teddy Riley, L.A. Reid, Missy Elliott, Lauryn Hill, Rodney Jerkins, Alicia Keys, R. Kelly, Wyclef Jean, Akon, Swizz Beatz, Robert Clivilles & David Cole, Jeffrey Osborne, Chuck Jackson, Jellybean, and others.
WalterAfanasieff, who played a significant part in Mariah Carey's career, also worked with Whitney Houston.
Gracing the covers of Ebony, Jet, Vibe, Black Hair…these publications called her R&B/soul. She graced the cover of Jet magazine 30 times alone, and her songs made Jet's R&B/Soul chart several times (like Mariah Carey). She made mainstream publications such as the All Music Guide To Soul, All Music Guide To Rock, The Virgin Encyclopedia of R&B and Soul and other reference works.
She made the cover of the acclaimed, long-lasting Blues & Soul magazine at least two times and has been treated as R&B/soul…again, Blues & Soul. Whitney Houston made the Top 100 Soul Albums of All Time and "Saving All My Love For You" made the Top 100 Soul Singles of All Time. They also voted her among the greatest R&B/soul artists of all time.
Hip-Hop Weekly certainly called her R&B/soul, as have other hip-hop magazines.
Rolling Stone: "Whitney Houston has one of the most powerful and polished voices in popular music…capable of balladic intimacy, gospel exuberance, and soulful expression."
Rolling Stone: "Redefined the image of a female soul icon and inspired singers ranging from Mariah Carey to Rihanna."
Life magazine: "She had opened the door for others. Janet Jackson's and Anita Baker's careers owed a sizable debt to Whitney Houston back in the day, and so do, today, those of Rihanna and Beyonce."
New York Times: "Houston was a major catalyst for a movement within black music that recognized the continuity of soul, pop, jazz and gospel vocal traditions."
New York Times: "R&B's great modernizer…slowly but surely reconciling the ambition and praise of the church with the movements and needs of the body and the glow of the mainstream."
Essence, a black magazine: "There would be no Mariah, Beyonce, Christina, Alicia, Jennifer or Adele if there had not first been a Whitney"
Essence ranked her fifth on their list of the 50 Most Influential R&B Stars of all time, calling her "the diva to end all divas."
Ebony, a black magazine: "R&B diva Whitney Houston makes her acting debut in The Bodyguard"
Ebony also voted "Greatest Love of All" and Whitney Houston as among the greatest R&B/soul artists of all time in 2002.
Rev. Al Shartpon: "I think Whitney had one of the most gifted voices that we have heard in our lifetime and she sang not only at people, she sang with and to people. She learned in the black church how to connect with people's hearts and she perfected it and polished it in a way that she brought the soul and great singing in our community to a global level and she represented it well."
Rhythm & Blues Foundation, 2/12/2012: "Whitney Houston descended from music pioneers, learned from them, and transcended even their highest peaks. Using her gospel and R&B roots, Whitney added pop overtones and current trends to create performances and set standards that may never be equaled. We will all feel the loss deeply, and the Foundation joins family, friends, and millions of fans in mourning her."
Just some random quotes from the hundreds and hundreds I have on her:
Bonnie Raitt: "She broke the glass ceiling for female R&B singers to cross over"
Wyclef Jean: "Every girl wanted to sound like Whitney, look like Whitney, dress like Whitney"
Aaron Neville: "Whitney had such a beautiful and controlled voice…she has her own soul"
Usher: "She is the diva. She is the soul singer"
Luther Vandross: "She is an original talent. She is a pioneer."
Faith Evans: "That voice was like 'soul, soul, soul."
India.Arie: "She was excellent on so many levels, and set the standard for what it is to be a great female artist."
Corinne Bailey Rae: "I think Whitney Houston was an influence on all singers, really, that are sort of my age, and for those younger as well."
CeCe Winans: "She showed us that you could reach heights that people have never reached before…Whitney had definitely opened a lot of doors."
This is the same woman where Cissy Houston is her mother (who had significant contributions to R&B/soul music), Dionne Warwick is her cousin, and her godmother is Aretha Franklin (Darlene Love was also Whitney's godmother).