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I actually find it very hard to call.
The first game is a classic. It forged the template for modern cover-based shooters, set a new bar for video game graphics, and created a gritty world with fearsome foes and a badass sense of style.
Gears 2 was not as original or innovative, but it refined what made the first game great, and upped the ante in terms of scope.
Gears 3 then followed the same pattern, not really doing anything new, but improving what had come before.
From a purely mechanical standpoint, 3 is the best, as it benefits from the most refined version of the core gameplay, but at the same time the first game's impact at the time makes it arguably more of a standout achievement, while the second has the most memorable setpieces of the three in my opinion.
