I really enjoyed all of them through
Spectre. That's where the series ends, in my personal universe.
I wish more authors of today's books took at least
The Ashes of Eden into account when mining for continuity. There's nothing in it that inherently contradicts any later canon, or anything else in the novelverse AFAICT. It was nice to see an acknowledgement of it in
Cast No Shadow.
The Return is a decent book in and of itself, but actually
Avenger is my favorite of all the Shatner novels.
Spectre was good, too... but then came
Dark Victory (*shudder*).