you are happy is most important

给楼主提供几种说法吧，
1.主语从句
That you are happy is the most important.
2.it引导倒装句
It is the most important that you are happy.
3.用名词形式
Your happiness is the most important.

另，重要的可以用：
important,fatal,essential,main,basilic,concernful,crucial,momentous,significant,staple,vital
等等来表示，

其实你想表达的是，我最在乎你是否开心，
即，Your happiness is what I care most.
希望能够帮助楼主，祝你成功！

Your happiness is the most important thing!
注意一个句子的主语与谓语的用法 ，一般一个句子只有一套真正意义的主谓宾结构，什么样的词可以做主语和什么样的词做谓语都要注意！！

上面两个都不对，应该是
Your happiness is the most important of all.

Your happiness is the most important! punkpiao的不要再用机器翻译害人。

Your happiness is the most important thing.

the most important is your happiness