Forms are needed on almost every site. Contact forms, registrations, orders, surveys. Gravity Forms and WPForms are the two most popular paid options, and they solve the same problem in quite different ways.
Gravity Forms
259 USD per year for the Elite license. The interface is functional but looks a bit dated. That's made up for by the functionality. Calculation fields, multi-level conditional logic, payment integrations (Stripe, PayPal, Square), and a webhook integration that makes it possible to connect the form to virtually any external system.
We use Gravity Forms in projects where the form is more than a contact form. Quote requests with calculations, registration forms with file uploads and conditional routing, booking forms with date selection.
WPForms
199 USD per year for Pro. More modern interface with a drag-and-drop builder that's genuinely pleasant to use. Faster to set up simpler forms. Has most features Gravity Forms offers, but the advanced logic and calculation capabilities aren't as deep.
Free alternatives
Contact Form 7 is still an option. Free, lightweight, but requires HTML knowledge for customizations. WPForms Lite provides basic functionality at no cost.
Our rule of thumb
Complex forms with calculations and integrations: Gravity Forms. Simpler sites where the client should be able to modify forms themselves: WPForms. Minimalist contact form: Contact Form 7 still does the job.