Hi Abraham,I think you can use the bases on the SIESTA site, any of them should give you better results than unoptimized ones.
But I would suggest using the one with the biggest cutoff radius.You can check which one would work better for you by comparing the total energy for calculations with different bases - the one that gives the minimal Etot is the best suited for you by variational principle. Check also the computation time, but I don't think it would depend that much on the basis.
