If I were you, I would say sorry to him.
This is because we always use "would" to express something that are out of supposition. That is, I am not you, so "supposing I were you". This is the meaning of supposition.
2008-08-07 22:41:52 補充:
Please also refer to my answer on the following question :
雖然If I were you 個'If'係用係將來式,但係喺呢句句子入面係仲有個過去式既were,如果if同were係同一句句子到,就會係用past tense,因為if同will都係將來式,好容易令人混淆呢句係要用將來式既,不過if如果係用係past tense入面,都唔會有任何既影響,句句子仍然係past tense。
would係will既past tense,so其實成句句子係寫I would say sorry to him.