I'm sort of your opposite: in my original readthrough, I managed to completely miss this one, and only read it after Cryoburn. This is the book that made me get all the fandom's interest in Gregor (and there should be more).

What's hit me with all the rereads is how it's all connected together – all the offhand mentions foreshadow something larger, things click into place way earlier, that sort of thing. Also, Gregor needs a hug.

I'm tentatively placing this book as #2 after my most favorite, Cetaganda, though let's see how well that holds up on a reread.