...in terms of impact, as how the general public sees it, as how the critics sees it and count in your personal opinion.

Decide.

Beautiful, Genie In A Bottle, Hurt, You Lost Me, the entire Stripped album >>>

Fighter, Can't Hold Us Down, Beautiful, Come On Over, Genie in a Bottle and Hurt.

Beautiful is her most memorable.

But remembering those hits:
Genie In A Bottle
What A Girl Wants
I Turn To You
Come On Over
Nobody Wants To Be Lonely
Lady Marmalade

I miss the old Xtina I think those oversexed songs & image from Bionic made the public lose interest to her. I know I did

Genie In A Bottle
What A Girl Wants
Come On Over
Nobody Wants To Be Lonely
Lady Marmalade
Beautiful
Fighter
Can't Hold Us Down
Ain't No Other Man

Genie
Beautiful
Come On Over

If I had to pick just one song that comes to mind when I hear her name, definitely "Beautiful".

But I picked: Genie, Lady Marmalade, Beautiful, Dirrty, and Candyman (on accident ).

Genie, What A Girl Wants, Beautiful and Dirrty...

Genie In A Bottle
Come On Over
Lady Marmalade
Dirrty
Beautiful
Hurt

Dirrty, Beautiful & Hurt.

Hurt is such a deep & relatable song.
When you try to make someone feel bad but end up suffering in the process...

Genie, WAGW, Come On Over Baby, Lady Marmalade, Dirrty, Beautiful, Fighter, Ain't No Other Man and European #1, Hurt.

The only ones that I think are that memorable to the general public are Genie in a Bottle and Beautiful. But there could be more. I'm always stingy when it comes to "signature hits."

For me: Genie in a Bottle, What A Girl Wants, Beautiful, Fighter, Aint No Other Man, Hurt. Dirrty.

genie, what a girl wants, and beautiful are her signature songs.

Idk why "Not Myself Tonight" has so little votes, but that is most definitely a signature.

What marked the beginning of Xtina's decline??

too many breaks and too many drastic changes per album era for fans to adapt to.

and Bionic could have did well but the single choices were horrid.

Beautiful, Genie, Lady Marmalade, WAGW, Dirrty, Fighter and IDK, But Candyman Was really Big Here , all my friends know how to sing and remeber the MV until today.

Fighter
The Voice Within
This is the best Christina songs ever.

Also, I voted for Beautiful and Hurt. Her ballads are flawless, except You Lost Me.