Which tool is most suitable for cutting thick cardboard with ease? I have a big scissor but cutting very thick cardboards like  tv case  in different shapes like butterfly wings consume too much effort.

What kind of tool will make my life easy w.r.t very thick cardboards?
My preferred tool for heavy card is the classic Stanley 99 knife.  The blade is slightly stiffer than those of the snap-off craft knives, and the metal handle is good and solid.

If you do go for a snap-off knife, for heavy card stick to one that takes 18&nbsp;mm or even the rare 25&nbsp;mm blades, rather than 9&nbsp;mm blades, which are too flexible (but great for finer work on thin card).  The cheapest of these 18&nbsp;mm and 9&nbsp;mm knives are rather flimsy and don't latch very well, so it's worth buying a good one (probably sold singly rather than in a big pack).

You should buy plenty of blades, as card can blunt them fairly quickly and a blunt blade tears the workpiece more easily than a new blade.