I used to be a proponent of NSS, when people who were much more closely involved with TLS implementations were quite down on it. NSS was the first TLS library I really read "cryptographically" (it hosted the Bleichenbacher e=3 signature verification bug, which made Firefox vulnerable to signature forgery) and I found the code really easy to follow, unlike OpenSSL's. Ultimately, they were right, and I was wrong: in this space, you want a lot of attention and effort on your TLS library, and OpenSSL is the most seriously maintained TLS library. For the same reason: OpenSSL over LibreSSL.