The most advanced, comprehensive forms plugin available for WordPress.
Get this plugin if you plan to integrate medium-complex level forms, or you are unsure of what your forms requirements may be in the future as it will handle any future requirements with ease.
The advanced nature of this plugin is not immediately apparent until you start using it - very extensible, and the support team always respond To questions within a few hrs.
I have been using this plugin for 2 years now and it still surprises me on what I can achieve with it. Advanced customisation requires a mid-level WordPress programming experience, though there are plenty of examples and help available within their support network if you get stuck.
Completely customisable via CSS and custom HTML. Multi-column layouts and sliding panels. Free addons for registered users include Paypal checkout, mailchimp intragation, and automatic Post and Taxonomy creation.
The only negative is no Ajax support, though this is supposed to be coming soon.