"We Found Love"
The simplicity of the lyrics are highly effective. The repetition of the hook illuminates the mind of listener. The production and overall structure isn't that of a typical pop song. It was something fresh, and not entirely predictable of her as a recording artist. Better yet, the music video visually brings these key ideas to life with highly modal imagery. It's making a point. For me personally, "We Found Love" is more iconic on most aspects.
I think both are iconic for their own time, though Umbrella is a song that can be played anytime, while WFL is more based around the time it is released.