Superlatives with 'the' 1

Fill the gap with 'the' if it's necessary. If you can choose either, then use 'no article' (-).

1) Everest is highest mountain in the world. [ . ]
2) Who is oldest person in your family? [ . ]
3) This dress was cheapest. [ . ]
4) Which language do you think is easiest to learn. [ . ]
5) This book is most serious one on the topic. [ . ]
6) I think that one over there is strongest horse. [ . ]
7) This film is shortest. [ . ]
8) She's fastest runner in her school. [ . ]
9) That suitcase is lightest. [ . ]
10) Out of all the cities in Europe, London is biggest. [ . ]