An Australian nurse who worked with dying patients over many years compiled what she learned into a book, but it can be summarized here:
The Top 5 Regrets of the Dying:
1. I wish I'd had the courage to live a life true to myself, not the life others expected of me.
2. I wish I didn't work so hard.
3. I wish I'd had the courage to express my feelings.
4. I wish I had stayed in touch with my friends.
5. I wish that I had let myself be happier.
I find the 1st 2 the most fascinating. There is a good summary article here: Top 5 Regrets of the Dying
As well as her book you can find here: Top 5 Regrets of the Dying By Bronnie Ware