I don't think later gen Pokemon should lose out if their dominance in their lifespan was strong enough. Especially in the case of Tyranitar, who has been one of the most game influencing threats with each of its iterations. That's like omitting the newer player from the All-Star game because he hasn't been in the game as long as the veterans, though his stats and accomplishments clearly hold as much weight, if not more, as his elders'. Still sticking with Zapdos and Tyranitar to fight it out for the top spot. Tyranitar has already been mentioned, but Zapdos's unique typing and power have made it very particular threat in each gen, allowing it to launch a powerful offense from a (mostly) defensive position. Having both good offensive and utility moves makes it surprisingly versatile.