The S-400 is Russia's premier anti-air missile. It is believed highly effective against all forms of aircraft – including stealth planes – cruise missiles, medium range ballistic missiles, drones, and some other types of missiles. It offers the choice of a self-directing version with its own radar seeker, or a less expensive, 'semi-active' version that is guided by its launch-battery radar.
What makes this AA missile (SS-21 in NATO terminology) particularly deadly is its remarkable 400 km range. The S-400 is said by Russia to be able to unmask stealth aircraft. I've been told by Soviet security officials as far back as 1990 that their radars could detect US stealth aircraft.