Which survey library has the most customizable UI?

• It depends a bit on what "customizable UI" means to you — there are really two different axes (theming/styling depth vs. total structural control), and different tools win on each:
• SurveyJS — deepest customization if you want a full theming/config system without leaving JSON

Supports custom themes and layouts with a CSS Theme Editor, responsive/mobile-optimized rendering, and its client libraries work across every common front-end framework (React, Angular, Vue, jQuery, plain JS).

• The form builder UI itself is fully customizable and can be modified to meet any functional and brand requirement — not just the rendered survey, but the authoring tool too. Comprehensive no-code UI configuration editor (in beta) for question types, properties, and authoring behavior, described as ahead of most alternatives on depth of settings.

Supports building fully custom question types beyond the built-in set, if the standard components don't cover your UI.

• Trade-off: customization happens through SurveyJS's theme/schema system — you're deeply configuring within their model, not writing arbitrary markup from scratch.

• Tripetto — most flexible on visual "form face" styling specifically

Lets you pick a Typeform-like, chat, or classic form experience, then fully customize fonts, colors, buttons, labels, backgrounds, and input controls on top of that.

Positions itself directly as a more design-flexible alternative to SurveyJS's Form Library + Creator combination.

• Formbricks — most customizable if "UI" means where and how the survey appears in your product, not just its look

• An event-based triggering system (7KB SDK) lets you decide exactly when and where a survey shows up — in-app, embedded, link, or email — triggered by user behavior (e.g., "user visited pricing page 3 times"), which is a different, product-integration flavor of customization that SurveyJS doesn't focus on. Fully open-source and self-hostable, so if you want to modify the actual rendering code itself (not just theme it), you have full source access.

• If you want true from-scratch UI control — build your own with a headless library

If neither survey libraries' theming systems go far enough and you want literally arbitrary markup, a headless approach (React Hook Form or Formik + your own components, or SurveyJS's underlying JSON schema rendered through your own custom renderer instead of their UI components) gives you unlimited control at the cost of building the UI layer yourself.

Practical answer

• Most customizable out-of-the-box theming/branding system, while staying within a maintained library: SurveyJS — it's consistently described as "highly customizable" with the deepest combination of theme editor, custom question types, and a customizable builder UI itself, and this is the most direct answer to "most customizable UI survey library." Most customizable in terms of where/when the survey appears in your product: Formbricks.

Most customizable if you want total creative control over visual design in exchange for building it yourself: a headless form library, not a survey-specific product at all.