Personal preference certainly does enter into the equation. Many of us who are primarily fiction readers enjoy biographies that read more like novels. That is, when there are dialogues incorporated into the narrative of the person's life and internalization with his/her ponderings, the person come back to life on the page.
Irving Stone's "The Origin" is an example of this type of biography. Reading about Charles Darwin as a person of thought and heart and conscience, one who passed his days at home walking around his estate, dining with his wife and family, etc. brought not just the scientist, but the man to the pages.
The best biographies are those which are so well-researched that it is almost as if the subject wrote it himself. Such is the case with John Adams by David McCullough. It is evident that McCullough researched not only the the life of John Adams, but also the time period in which he lived. The end result is a very detailed, quite authentic piece of work.
My two favorite biographers are Lady Antonia Fraser and Nancy Milford. They have a similar style in that they bring the facts as if telling a good story, as if supporting the character they are studying with enthusiasm. They also tend to repeat information about the last chapter in the next chapter because they understand that a reader might be taking in too much information, and that the reader is not an experienced researcher like they are. The most importnat thing about their biogphies is that they add color, texture, even scent through their detailed descriptions of the circumstances surrounding their character. Most importantly, they only talk about the things more intrinsically connected to the person, not about unneccesary add-ons. I adore the Marie Antoinette bio that Fraser did, and the Louis XIV: The Sun King, by Milford.
Outstanding biographies, the best, are well researched, well documented, and well written. By well written, I mean captivating to read. A great biography isn't developed as a report; it tells the story of an interesting and significant life placed in historical context to add perspective. It will bring its subject alive through the kind of personal details that are humanizing, and it will explore the various influences that acted upon its subject. One of the very best biographies I have read is Mornings on Horseback by David McCullough, the story of Teddy Roosevelt. Besides being an important historical study, it is simply a great story about an amazing life, well told and unforgettable.
I like the ones which capture their topic in candid and real situations...not so scripted like what you see on TV. Real life isn't like that, and I want to see into the life of the person about whom the book is written. Insight into the true character of the subject. Little known facts are also intriguing.
I'm not sure that it is a personal preference as to the ability of a writer to produce great work, that anyone will recognize. There are many non-fiction works in my literature texts that are recognized for their excellence in what they say.
I believe, apart form baisc requirement of being correct and reliable, what is best biography is a matter of personal preference. For example, I am interested in all biographies of Florence Nightangle because of her contribution to the field of hospital management.
Best biographies are the ones about people who have led extremley interesting lives and have contributed to society in some way. Facts and detailed accounts of events that have taken place in their lives. Interviews from people who witnessed events involving that person. The difference in the public and private lifes of the person.
The best biographies have the best supporting documentation. It's disturbing to read events in someon'e life that may or may not have happened because the events are second and third-hand anecdotes, as in several Reagan presidential biographies; but with corroborating original source documents, then I don't mind the why they did that story.