It's hard to even say, the only real girlbands around atm are Little Mix, The Saturdays and MKS. The latter is barely in the running, and The Saturdays and Little Mix are at very different points in their career to even compare. I wouldn't even say they're in competition with one another. Just for the larger back catalogue which has been some great pop for the most part, I'd pick The Saturdays.
Girls Aloud and Sugababes are gone now