What I think of as the core of the series is the run of books from Deadheads (1983) through Death's Jest-Book (2003), which are almost all excellent. The problem is that Deadheads, while great, is not really typical of the series. So maybe start with Child's Play (1987), which has both a powerful mystery plot and some major character development. If you like it, read on chronologically from there, and maybe loop back at some point to catch the earlier books.