Daniel J. Bernstein actually regards the 521 curve as the only secure NIST curve, asserting that the 256 and 384 curves have too many suspicious and unexplained terms.

"To be fair I should mention that there's one standard NIST curve using a nice prime, namely 2^521 – 1; but the sheer size of this prime makes it much slower than NIST P-256."

http://blog.cr.yp.to/20140323-ecdsa.html