Second top I guess
Why is the puzzling question and Karan have given some insigts as to why IAF is so keen on this long range SAM even after so many SAM systemss under induction or in pipeline.
Kuch to baat hai sir!
S-400 to my mind has 3 main advantages
1. Flexibility - a single missile system with missiles with multiple ranges and able to handle a variety of targets - aircraft, cruise and even IRBM class. This means a single S-400 system can effectively guard a set of vital targets by putting a shield around an entire vital area against a range of targets.
2. Very high survivability - the system is highly mobile and can be brought into operation and relocated very quickly. This makes hunting it very hard for even USAF level operators.
3. Very high (theoretical at least) effectiveness. The S-400 uses a derivative of track via missile guidance. The missile sends back target data to the command center which then compares target data from the missile and the radar and uses what is more accurate. So conventional SPJs which jam the missile seeker won't work. Jamming the huge main radar, easier said then done, since it has many advanced features and waveforms plus algorithms to detect and ignore jamming. Next, the S-400 power output was considerably increased above S-3XX so merely blanking the airwaves with noise won't work either. The datalinks are pencil thin and directional. So to jam all three radar, datalink, missile seeker you have to put in a huge EW effort and even then, there is nothing to say that the S-400 battery is not getting cued by another battery or extra radar co-located with it. Otherwise, you have to spam it with decoys, somehow con the missile operator to use up his rounds and attack the system. All in all, the S-400 is a huge challenge for a very well-supplied opponent. This is one battery. Now imagine 5 regiments, each with multiple independent batteries (search radar, fire control radar, missiles, command post).
The combination of all 3 attributes makes the S-400 a very hard nut to crack and also, very valuable to any AF.
One can only imagine what will happen if Russia starts producing GaN radars with very high power output allowing these systems to detect VLO targets at range.