le plus... que

Example

C'est le plus beau voyage que j'aie fait.

Why

After a superlative, the relative clause takes the subjunctive: you are giving an appraisal over the whole of your experience, not a verifiable fact. Note the form aie, first person subjunctive of avoir. The indicative is found when you state a verifiable fact rather than appraise it, but the subjunctive is the form expected in an exam.