This topic has been done many, many times.
IMO, Vito was the biggest fish in a relatively small pond: NYC Mafia in the pre- and immediate post-WWII era. He succeeded in controlling his pond. BUT: he lost his oldest son, and he failed to achieve his dream for his youngest son (to be a Senator or Governor).
If Vito fished a pond, Michael rode the ocean: he was the biggest gangster in America and possibly the world. But he failed to become totally legitimate ("just when I thought I was out, they pull me back in"); his surviving brother betrayed him, his wife left him, his son wouldn't follow Michael's plan for his life, and his beloved daughter was murdered before his eyes. Michael died alone, attended only by a little dog.
Who was a better Don? Neither succeeded in achieving the lifetime goals they set for themselves. Michael, especially, spent his life winning battles and losing wars.