I build a LOT of forms and one of my favorite extensions to use is RSFormsPro! from RSJoomla. With all of its integration points, layouts, field types, and ability to add custom overrides, I end up using it, well...a lot. They've just released version 1.51.2, which is their best to-date. This release increases the number of responsive layouts - perfect for responsive - sites with the introduction of Bootstrap 2 , Bootstrap 3 and uikit (useful if you are using YooTheme templates) options.
If you've not used RSFormsPro!, there are a few samples to get your started, documentation, forums, and good support. Got a complex form with conditional fields and other advanced features, most likely RSForms, can handle it.
Try it out for yourself. You can read more about the new features at: https://www.rsjoomla.com/blog/view/375-rsformpro-got-even-better-version-1512-is-out-.html.