So many, from family members (mother, father, grandparents, etc.) to people I admire who have touched my life from a distance, such as spiritual leaders, musicians (Mozart!), artists, etc.
To all I would say: "I love you, thank you, what did I get, what did I miss, what should I have heard or seen but did not, what would you change, what and who did you love, may we meet again."
No friends have passed away yet, but to a little boy in my class in grade school who was very intelligent, quiet, and nice and who was friendly to my sister and to me but who ran away one night from an apparently unbearable home life and who was found frozen to death under the porch of the house across from our house, I would say, "I remember you, and I feel sadness that your life was so short and so filled with pain and that you were alone."