need help

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?

...R

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.