Who is the most appropriate person to recite the kaddish at the end of the Torah reading, the gabbai or the baal korah and why?
It is more appropriate that the Baal Korah should recite the Kaddish.
The reason for this is that the Kaddish follows on from the Torah reading, and therefore just as the Baal Koreh reads from the Torah, it is appropriate that he should say the Kaddish. This is similar to the regular Kaddeishim that are part of the Shacharis (and other) davening: The Chazan recites them, just as he recites the rest of the davening.
See Mateh Efraim, Kaddish Yatom 3:1; Halichos Shlomo 12:27; see also Daas Chazon Ish 2:18 where the option of an aveil reciting the Kaddish is preferred.