What it says on the tin. Plotwise and Gamewise, which game did you think did summons the best.
Gameplay-wise, I think X did it well, because it actually had them on the field with you while keeping the nukiness from previous games. Also, summon meatshield saved me too many times to count
Plotwise, I'm stuck between FFVI, FFIX, FFX and FFXIII. I liked the idea of magical beasts and them being the 'MacGuffin' the villains were searching for, but I also liked the idea of the pilgrimage and Fayth in X, and the emotional nadirs of XIII (although some of the 'summonings' in XIII were just random