I am trying to use only the strongest ciper suite available, from the documentation I tried:
But the browser says that it is obsolete:
What is the strongest one available at the moment?
THE strongest is perhaps not well determined. But Chrome wants to see something like TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256, i.e. the cipher suite has to use DHE or ECDHE and AES in Galois Counter Mode.
Alternatively you can use CHACHA20_POLY1305 as the symmetric algorithm.
IETF recommends the cipher suites
in RFC-7525 section 4.2 as of May 2015, which I would say are a good choice.
However, the EC variants almost always use the NIST curves (secp256r1, secp384r1, secp521r1) which are subject to speculations as described here.
Note that the strongest available cipher suite is not determined by Apache Httpd; it is dependent on the version of openssl (or any other ssl implementation) or your operating system. There are still operating systems coming with "oldstable" openssl-0.9.8 :-(