What is the best solar system application to secure money in short time .( on grid , off grid , solar heater , solar pumping , solar street lighting )?

Any application that avoids the usage of batteries would be cost effective; since battereis are really expensive compare to other elements. The invention of grid-connected systems is mainly to avoid the usage of batteries and reducing the system cost. 

solar water heater is the best to secure money in short time, because many reasons one of them is the installation cost which is low comparing with other systems and the second is the high saving, so the result will be the lower baybach period.o grid system also good but couldn't work as solar heater effecient. 

On grid systems benefit customer by providing savings in terms of electricity bill. These savings provide for the payback for the initial cost of these systems. Based on the performance of on grid system, if it is able to provide large savings by being functional throughout the day customer can secure the inital capital and acheive monetary benefits in a short time.

Solar Water heater and on grid system also solar pumping if you are in a remote area

solar heater as its r.o.i. is around3-5 years.