Which material to use for sealing (as gasket) the joint b/w the exhaust port and the silencer?

I have used Silicone Gasket Maker... (Translucent color) Works Good.
Have also used Anabond Gasket maker (red color) - this is also good.

Normally not required unless the surface of the mounting flange has been damaged.

Get high temprature gasket sheets from your local auto market, cut out gaskets and hey presto you have tight non leak joints.
The trick to cut out the gasket is use the old stamp pad. Make an ink impression on the gasket with the muffler and you can easily cut it out.
Gaskets can also be made out of thin and soft aluminum sheets in the same manner.

Thanks all for your precious advice. I got Anabond Gasket maker (red color), High temp. silicon RTV from a local hardware shop. I think this will be perfect for my car engine rather than a sheet type.

i also used it on that joint i thought it was done.....the same rtv silicon sealant red ...