I took the 2 weakest from each year this decade, based on their 2nd and 3rd strongest selling weeks. Cuz...a big opening/Christmas week doesn't make a song a strong seller. There have been many grumblings about how quick the falls of Part of Me and Diamonds were, for example.

2010 Rude Boy 175, 171
Only Girl 161, 157

2011 S&M 178, 173
Hold It Against Me 183, 133

2012 Diamonds 176, 171
Part of Me 187, 142

2013 Harlem Shake 262, 228
When I Was Your Man 240, 235

2014 All of Me 226, 224
Rude 210, 207

2015 Cheerleader 172, 170
Can't Feel My Face 148, 134

I think all of them are songs that people around here would agree weren't huge in the US.
I'm going with HIAM and Rude.

Born This Way sold over 500k in it's second week and over 300k I think in it's 3rd week.

Diamonds & Rude Boy are my favorites.

all of Rih's + CFMF + Rude

Rude Boy is probably my favorite out of all of them, though.