Lean On, can't think of any others.

Wrecking Ball
Motivation
Where Have You Been

Rihanna rejected Lean On, at least according to Diplo.

Céline: I Don't Want To Miss A Thing

Yes.
- Titanium
- Elastic Heart
- Bad Blood
- Pretty Hurts (well, that was Katy's mistake.)
Don't remember if there are others.

Michael Jackson rejected La Isla Bonita

pink has rejected since you been gone and that just a fool song whch was actually written for her.

My faves write their own music so yeah, they can't reject their own work

b. what @ all of these?
I don't think WB is the Dr. Luke song Bey rejected and I don't think it made Miley's album either.
---
I Got Your Back w/ TI [so I've heard]
Find Your Love by Drake was originally for her
All About That Bass
Baby I
Only 1 on Ari's album was also for Bey

Rihanna: Lean On. But Come & Get It, We Cant Stop, and Pretty Hurts were all written for the good sis.

The only ones that I know of for Shakira are:

We Found Love

Give Me Everything



She's also reportedly worked with a bunch of artists/producers but the songs never came to light - these include Usher, Skrillex, LMFAO, Akon, NeYo, and a bunch more.

We Found Love
Give Me Everything

I can't picture Bey on "Baby I"

Demi:
Heartbeat Song.

Pretty Hurts
Titanium
Elastic Heart
Bad Blood

Gaga writes all of her own music but I'm sure some people attempted to offer her songs.

Nicole and Leona also rejected WFL.