Vito's always the sentimental favorite on these boards--"nice," "family man," "grants favors," etc. Michael is "cold," "ruthless," "killed his own brother," "lost his family," etc. Strip away the sentimentality and...
--Vito was a big fish in a relatively small pond (NYC Mafia in the Forties). Michael was a whale in the world's oceans: he was the top Mob guy in the country, was maneuvering to take over gambling in Cuba, and even in III, when he was supposedly "out," was moving to control the Vatican Bank and Int'l Immobiliare. He big off a much larger chunk than Vito.
--Michael constantly won battles but lost wars, losing his brother, his wife and his beloved daughter, died alone, attended by a dog. All because of his lust for power. But Vito lost his oldest son and saw his youngest son, on whom he placed his hopes for family legitimacy, dragged into the Mafia because of Vito's mistakes and actions.
I say neither of them was an "ideal" Don.