It's extremely hard to judge some of these newcomers, most of which have a few shows under their belts, and try to compare them to composers who have written 10-15 shows - but I will try to examine the work itself - the work on paper. It's hard to draw the line between what I enjoy the most and what is the best written. But since it says favorite, I'll try (for once) not to examine too deeply.
So, basing my opinion on srictly the material...top 10
Stephen Sondheim
Adam Guettel
Richard Rodgers with both Oscar Hammerstein II and Lorenz Hart
John Kander and Fred Ebb
Leonard Bernstein with Comden/Green or Sondheim
Jeanine Tesori
George and Ira Gershwin
Cole Porter
Jason Robert Brown
William Finn
honorable mentions in no order:
Jule Styne with Comden/Green or Sondheim
Alan Jay Lerner and Frederick Loewe
Michael John Lachiusa
Kurt Weill
Jerome Kern with Hammerstein II
But I won't live alone in a house of regret.