I have several "mains," as it were, to the point where "maining" a Frame has lost its meaning.
I play Excalibur, Atlas, Ember, Frost, Vauban, Hydroid, Octavia, Oberon, Saryn, and am currently looking to get Mesa and Nidus. I thoroughly enjoy all of them, but I would say that Vauban represents me most, since he was one of the earliest Frames I went for, and I grinded to get him legitimately.