Transition meaning the song they became a somewhat different type of artist, took their music in another direction or slightly changed their genre of music. This can also mean appealing to a different audience or gaining confidence.
Here is the list
Britney Spears - I'm A Slave 4 U
Christina Aguilera - Dirrty
Beyoncé - Crazy In Love
Rihanna - Umbrella
Mariah Carey - Honey (Bad Boy Remix)
Brandy - Afrodisiac
Nicki Minaj - Starships
Sean Paul - Other Side Of Love
P!nk - Get The Party Started
Kelly Rowland - When Love Takes Over
Cheryl Cole - Heartbreaker (I know it's Will's song but this was her turning point)
This is all I can think of from the top of my head but tell me if I have forgotten anyone. Btw don't get mad if your fave isn't on the list, it means that they've stayed true to themselves (no shade @ the list)
I'm gonna vote for Mariah Carey because it was the original and most popular transition, not to mention that the song is amazing.