I think all HK singers speak mandarin well, cause almost all can speak mandarin of course them who lives in china xd But I will say Andy Lau xD
I wouldn't say that they speak Mandarin well...after all, you can definately tell that they weren't educated in mainland China. In fact, stars like Jacky Cheung, Andy Lau, Louis Koo, and even Raymond Lam (who is from Xiamen, China) lack the authentic Mandarin pronounciation. (I guess their Mandarin is so-so). Despite that, we can still understand them and the all other Cantonese stars who totally butcher the language (cough...cough...Charmaine S., Ron N., etc. ). But, so far the best Mandarin I've heard (in speech) from a HK star are Myolie Wu, Vivien Yeo, and esp. Jessica Hsuan. Even so, their Mandarin aren't as well-pronounced as Michelle Ye's (yes, yes, I know she's from mainland; I'm just trying to bring across my point).
Well, I saw him in an interview with Jessica Hsuan and Sonja Kwok once...and his Mandarin was WAY better than Sonja's. In fact, Sonja kept filling words she didn't know/remember with English. My, oh my!
in no particular order, andy lau, jacky cheung, nicholas tse, calil fong (dunno if he counts though) and...im stuck thats it. everyone else i find their mando is really funny ^^