If you ask about the setting, this is a classic, debated at length not long ago, most recently (AFAIR) here: What is your favorite tribunal, and why?
If you ask which book is the best one, your question is a little more novel.
I think it is uncontroversial to say that the books have gotten better with each edition. The 5ed books spend more space on story seeds and ambience, and less on dry historical fact. Admittedly, the dry historical fact was more relevant when less information was available on the Internet, but even so, it has to be adapted to stories, and each edition does a little better than the previous one.
Loch Leglean is the odd one out, clearly better than the original 3ed line, but bogged down in annoying graphics, which makes it hard to get a fair impression of the actual text. It feels more fragmented than the 4ed books, so I count it as 3½ed.
I cannot judge between the 5ed books without repeating the last thread.