I definitely don't recommend the Second Foundation Trilogy by the Killer Bs. It was well written (of course), but not good Foundation. It was inconsistent with the original and-- well, I won't post spoilers, but let's just say it introduced an idea which was a great SF concept, but completely inappropriate for the Foundation Universe.
Benford's book is, and there's no way to sugarcoat this, awful. He completely missed the point of Asimov's Foundation
The next two, though -- Greg Bear's Foundation and Chaos
and David Brin's Foundation's Triumph
-- are, I think, the best Foundation
since Foundation's Edge