After reading comments of serveral commercial and non-commercial form creators, such as "RSForm!Pro", "BreezingForms" and "Mad4Joomla Mailforms", I finally choosed JForms.

The result is: I really love this extension! It's simple to setup, easy to create even complex forms.

The only shortage is: it's not very friendly to web designers. The generated HTML code is complicated, and it is not easy to precisely control the width and height of "input" element.

But after all, it's a great extension!

Very easy to install and configure. It just took me 2 minute to make it work. So excellent work!

PS:
while many parameters are easy to configure, parameters for "Image Folder","Images","Titles", and "URLs" are a little inconvenient. But still an very good extension!