Rule reminder
In standard È...che clefts, the subordinate clause verb is normally INDICATIVE. However, the subjunctive appears in specific contexts: (1) After superlatives or restrictive expressions ('È la cosa più bella che abbia visto'). (2) In hypothetical or concessive clefts. (3) When the focused element is indefinite or unspecified ('Non c'è nessuno che capisca'). (4) Pseudo-clefts with subjunctive for emphasis or doubt. Mastering when to use the subjunctive in cleft contexts is an advanced B2 skill.
Complete: 'È la cosa più bella che ___ mai vista.' (What mood?)