1. 这是他最喜欢的书。

应该是这样：This book is the one (which) he likes most.

或者简单点的翻译成这样：This book is his favourite.

这样写不对。
应该这样写：
This is he favourite book.

不对，应该是this is the book which he likes most先行词应放名词后

如果想写成定语从句的话 可以这样 This is the book which he liks most.
或者用简单句：This is his favourite book.

the book is he likes most