To me, you are definitely not somebody else's shadow. You are unique in my heart. (比较口语化）
或：I see you as unique, not as someone else's shadow. （把次序换了一下,且变得稍微简练了些,但意思不变）
I love you as someone who is absolutely unique and without a trace of anyone else's shadow.（并不算直译,但意思还在,且比较华丽）
另外,ls二位的语法都错了.不过前面二位的说法改正成：You are definitely not the shadow of any other person, but the only one in my heart,虽不是最好的,但还是可行.
第三位基本上无可救药了... 但勉强改成：You are my most beloved, not a shadow of anyone else,倒也不错.