In which I ask for recommendations

In addition to reading more this year, a goal I'm doing well with, I've also been trying to expand what I read. In the past I could pretty much hang out all year in modern non-fiction (If I'm going to read something written about this century, it will probably be non-fiction), historical mysteries and children's classics and be happy but I'm trying to push myself.

My two goals are to read 1) more challenging books and 2) a wider variety of genres. This has been slightly easier now that I'm forcing myself to keep 5-7 books going at once. But one category that I keep coming up short is biographies. I want to read biographies, I think its good to read biographies and honestly, I've even enjoyed reading the ones I've read so far this year (The Story of the Trapp Family Singers, Abe Lincoln grows up, and my current one, Julia Child's My Life in France) but once I finish one, I find it hard to pick up another.

It might have something to do with my dislike of sad books or those with many deaths. Most biographies, at least full and realistic ones, have some level of sadness in them and most tend to end in death. So as understandable as that is, it also an immediately bullet point on the con list. But still, the point of a challenge is to be challenging so I press on. But I need help! What are your favorite biographies that I absolutely positively must read?