Making a paper prototype is the most enjoyable for me. The reason is that I can do imagine the UX and UI freely.
Simple definition of UX/UI designer levels
[MOIN] iOS Account Deletion
[FileThread website] Mid-fidelity wire-frames
[FileThread website] Squeeze idea
UX/UI를 위한 게슈탈트 이론 (Gestalt theory for UX/UI)