Umbrella

The song was initially offered to Britney Spears management, who declined, to which The Dream (song co-writer) and Rihanna reacted:

We thought, ‘Let’s save our friend.' 
I’m so thankful for it, I prayed for this song.

Per the Rolling Stone

In 2018, NPR ranked this as the #24 greatest song by a female or nonbinary artist in the 21st century, saying:

Rihanna’s status as a pop icon was cemented on her third album, Good Girl Gone Bad, specifically with this irrepressible single. The song examines a fairly basic tenet of romantic relationships with a nuanced vocal delivery that slowly ensnares you. Just try getting the hook, ‘ella, ella, eh, eh, eh,’ out of your head; its sinewy simplicity is inescapable. Rihanna wisely recognized how well that repetitive refrain would ‘workworkworkworkwork,’ to quote a later hit of hers. ‘Umbrella’ offers a cool confidence to which we can all aspire.

Me, my cousin Tricky Stewart [founder of Atlanta’s music production company RedZone Entertainment] and The-Dream wrote “Umbrella.“ [Former Def Jam chairman/CEO] L.A. Reid heard it through Island Def Jam’s Karen Kwak. Rihanna just signed to the label and they knew right away that was a hit for her. A week later, we were in Los Angeles cutting this record. This was the first big artist I’ve ever worked with, and I wasn’t even technically considered a vocal producer at that point. I was like a kid in a candy store! But once I get in the studio, I’m not thinking about anytime except for making sure I get the best performance out of the artist. And once I’m done with a record, I don’t get into the science of, “Man, this artist is going to be huge!” I just stick to the emotions.”

Co-Writer Kuk Harrell via Billboard

Also, producer The-Dream said to Billboard:

I felt in my heart at the time that I needed a friend like this. I wanted to also be that person for someone else. It took maybe 15 minutes to get out the song, it began like a rap freestyle and it just never stopped pouring, metaphor after metaphor. I was in a zone and in a feeling that wasn’t going to end until I finished what I had to say.