Best I've gotten from 2K summons is silver. Best I've gotten from SP summons is gold.
I was lucky enough with my first 10 premium summons back in December 2015 of getting Lilia, Celia and Nanaly (along with Monica, Cuterie and Robert), but the only other platinum I've gotten since was Zenobia from a platinum ticket on January 2017.
But I'm not doing premium summons yet, still too many units to level before spending the SCs there.