Luyện ngữ pháp - English Grammar in Use - Intermediate ((Unit 108: Superlatives))
Study these examples:
Longest and most enjoyable are superlative forms.
The superlative form is -est or most .. .. In general, we use -est for short words and most ... for longer words. The rules are the same as those for the comparative - see Unit 105.
For spelling, see Appendix 6.
Oldest and eldest