Caliban's War. I have a biased opinion about middle books in general as I find that they tend to be mediocre or even regressive. By my reckoning, whether right or wrong, having a weak middle tome sets a lower bar for the finale, or presents a starker contrast to notch up our appreciation of the full work. As I said, this is a personal bias.
In the case of [b:Caliban's War|12591698|Caliban's War (Expanse, #2)|James S.A. Corey|http://d202m5krfqbpi5.cloudfront.net/books/1317402997s/12591698.jpg|17606541], the story thread is a secondary offshoot of the events following [b:Leviathan Wakes|15023786|Leviathan Wakes|James S.A. Corey|/assets/nocover/60x80.png|13730452], focusing on the alien proto-molecule components that have fallen into the hands of human adventurists, rather than the main infection brewing on Venus. But this differentiation is not noticeable until the very end as the story-telling and plot-mixing is top-notch. CW improves on LW with additional and more interesting POVs and addressing LW's issues of gender bias and uninteresting monologues. The writers really amp up the suspense and sense of adventure in CW. Also, the narrator, Jefferson May, brings to vivid life the character Avasarala, making her, in fact, the highlight of the audiobook version for me. Avarasala is truly "potty-mouthed", rude and obnoxious, and quite loveable as curmudgeons go, and is apt comic-relief for the rest of the book's POVs, most of whom are uniformly somber.
As a middle book, CW does its job and raises expectations for the finale, [b:Abaddon's Gate|12591719|Abaddon's Gate (Expanse, #3)|James S.A. Corey|http://d202m5krfqbpi5.cloudfront.net/books/1352637335s/12591719.jpg|17606564]. Hopefully, the authors have not done too good a job and set the bar too high. I am expecting/hoping this series to be a 4/5/5 (see below for explanation). We'll see.
How to rate trilogies? A simple way is to concatenate individual GR ratings. For example, the first Dune trilogy I rate as a 5/4/4 for [b:Dune|234225|Dune (Dune Chronicles, #1)|Frank Herbert|http://d202m5krfqbpi5.cloudfront.net/books/1349105964s/234225.jpg|3634639]/[b:Dune Messiah|106|Dune Messiah (Dune Chronicles, #2)|Frank Herbert|http://d202m5krfqbpi5.cloudfront.net/books/1347771287s/106.jpg|3634570]/[b:Children of Dune|112|Children of Dune (Dune Chronicles, #3)|Frank Herbert|http://d202m5krfqbpi5.cloudfront.net/books/1348092992s/112.jpg|3634573], and [a:Robert Charles Wilson|27276|Robert Charles Wilson|http://d202m5krfqbpi5.cloudfront.net/authors/1224814586p2/27276.jpg]'s Spin trilogy is a 5/0/4 (I did not read the middle book), while I give 5/5/5 for [a:John Varley|27341|John Varley|http://d202m5krfqbpi5.cloudfront.net/authors/1346593830p2/27341.jpg]'s Gaia series. In absence of an outright trilogy rating, this seems to work well enough.