Children of Time
There’s lots of answers to this depending on taste, but you also get into arguments about whether such and such is space opera or planetary romance. Children of Time is hard SF the way a reader from the 1960s would have understood it.
I second this -- but, at the same time, it's such a shame that Tchaikovsky hasn't written anything else worth a damn, despite writing something like three novels every year!
His two most recent, Shroud and Service Model, are bloated, uninspired, and borderline unreadable. I guess he's now subject to that curse of established authors, where editors are scared to mess with their manuscripts and trim the fat.
I got about a quarter of the way through this one before I ditched it. The premise was pretty interesting but it read like textbook to me.