What do you think is the application which is most popular with hosting clients?

Depends on what is the application is applied for...

For resellers I would name WHM. Looks they are the best ones

Cost production is one of the factors that rule the vast world of the outsourcing tool. Therefore you should always remember that settling for quick and dirty solution will definitely lead to more cost than saving it.

This is even applicable to PHP coding practices too. When you are involved in building applications in PHP you should never ever settle for quick and dirty solutions. This quick and dirty solutions are never reliable because it contain poorly written, monolithic code which can never be used again also its intention is not clear for proper understanding of the application.