IE: A game that would cause you to run out and buy a console instantaneously, regardless of the existence of any other exclusives you cared about. Assume a console price of $399 USD.
For me, it would have to be Final Fantasy Tactics 2. A true sequel, on modern hardware, with improvements to the battle system, job system, and late game balance (looking at you calculators and Holy swordsman).
Edit: for the console fans, feel free to list a game which, if it existed on PC, you'd strongly consider buying/building at least a mid range PC just to play it.