The one that brought you the most enjoyment, where you'd want to live, advanced Trek the most by displaying a counterpart. Or just had the hottest babes.
Yesterday's Enterprise universe.
Edith Keeler lived universe.
Spock died at seven universe.
The Borg are everywhere universe.
Or another of your own preference.
For me it's the Yesterday's Enterprise universe, it's interesting to conjecture how one event generated so much change over the course of forty years. And was Picard telling the truth or lying to Garret about the Federation losing? Was the Federation actually winning, but Picard wanted to deceive Garret into volunteering to save the lives of billions of people?
I also wonder if Spock was killed at seven, and the Enterprise (and Kirk himself) still existed until the time of TAS, were all of Spock's contributions to the prime timeline completely inconsequential?