I need a best and cheapest sensor to do analysis and send a message to a client with connecting in wi-fi and will put inside a tank.
Best and cheapest rarely go together.
As you have not told us what you want to sense it is a tad difficult to help.
What sort of tank?
inside a tank
You can't use Arduino in military applications.
Adafruit has a lot of Arduino clothing products/ideas.
You should be able to place things in a tank top.
I would look at Raspberry PI…it’s has a 4 quad processor…spy stuff.