After some browsing on the net, it seems that there are basically only 2 good liners: a 737 and A320. Unfortunately both of them come with no FMC even if it is possible to add but both of them are for free.
I have tested the 737 that seems quite interesting even if it is not comparable to FSX version of 737. But it is 100% free and as far as I recall there are no such good products available for FSX for free.
The flight model for this plane seems quite good. It was initially very nervous but after setting up my joystick it responds quite well.
The biggest problem is the lack of an FMC. It is possible to buy an FMC from the same site that delivers the plane but until now I tested only a freely available FMC: X-FMC.
X-FMC is generic in the sense that it is customizable for a lot of planes like the 737 or the FMC. It is quite complete with initialization pages, performances pages, routes, SIDs and STARs.
However the route is quite unrealistic, not smoothed at all and the plane does not follow it 100%. In the 737 this FMC is not well integrated in the sense that it works by setting the HDG and the appropriate vertical mode instead of VNAV and LNAV. This is enough for me to say that it is not acceptable as it is quite far from a good simulation of the systems.
But I kindly suggest this FMC as a chep solution and for flying offline.
Next step will to buy and test the x737FMC that claims to be 100% integrated in the x737. With the incoming version of X-Plane, I prefer to wait before buying.