Work B*tch is not a signature song in any way. At least not right now, and I doubt it will be in the future.

Beyonce: Single Ladies, Crazy In Love, Halo, Irreplaceable, If I Were A Boy, Love On Top
Michael Jackson: Beat It, Billie Jean, Man In the Mirror, Thriller, Smooth Criminal
Rihanna: Umbrella,

Maybe my definition of a signature song is more narrow than most people's. I'm seeing at least 3+ songs per artist added that are not signature songs. I'd say for Beyonce she has Single Ladies and Crazy in Love.

Super Bass
Starships
Moment 4 Life

+1

It is ...Baby One More Time and Oops!...I Did It Again

But I agree, but WB ain't one yet! I'd rather say Toxic

Rihanna: Umbrella. We Found Love. Diamonds.
Beyonce: Crazy in love. Single Ladies

Pon De Replay, SOS, Umbrella, Take A Bow, Disturbia, Rude Boy, Only Girl (In The World), What's My Name, S&M, We Found Love, Diamonds ...

I'd say SUBG, BOY and Stronger are her most signature songs.
Other ones: BHTE, Breakaway, MI, AMLT.

Janet Jackson:

Nasty
Rythm Nation
That's The Way Loves Goes
Together Again
All For You
Feedback

MJ:
Rock With You
Don't Stop Till
Billie Jean
Thriller
Beat It
Bad
The Way You Make Me Feel
Man In The Mirror
Black Or White
You Are Not Alone

Madonna:
Holiday
LAV
Material Girl
Open Your Heart
Like A Prayer
Express Yourself
Cherish
Frozen
Ray of Light
Music
Hung Up


etc............ got too much faves to list