Even more controversially, the Foundation Trilogy should've been.
I agree the Foundation Trilogy should have stayed a trilogy. It's not like the last two books ended the series either (even if it ended Seldon's Plan, that just led to a bigger thing). That being said, Foundation's Edge and Foundation and Earth are good sci fi books (until you get to the long exposition with robots), they just aren't in the same spirit as the trilogy. I haven't read the two prequels, though, so I can't comment on them.
I thought
Forward the Foundation was a wonderful book. (I would rank it fifth among Asimov's fiction volumes, after the
Foundation trilogy and
I, Robot.) Unlike the other sequel/prequel novels that he wrote later in life, it seemed to serve a purpose - as an elegy of sorts for Asimov himself, in the person of Hari Seldon.
I also thought it was the only later Asimov novel to introduce characters as memorable as those in his early novels - particularly Wanda Seldon and Hari Seldon himself, who is both a different man from the one he seemed in
Foundation, and wholly consistent with his appearance there; it's akin to meeting Theodore Roosevelt the man vice Theodore Roosevelt the legend.
I haven't read
Mission of Honor or . . . whatever the most recent book was called, but I thought the later Honor Harrington books were at least the equal of the early books in the series (certainly of at least all of them except
The Short Victorious War).
The Shadow of Saganami and
Storm from the Shadows, were especially a return to form, as, I thought, was
At All Costs - when not busy detailing the relative throw rates of missile barrages.
For my own taste, the
Ender series has grown too expansive. (Yet I await every new novel.) The strength of the series seems to be diluted with every new installment, with the possible exception of
Shadow of the Giant, which profited from a fortunate intersection with the epilogue of
Ender's Game that elevated the novel above other recent books in the series.