Yeah... too many people have tried this one... imho, the most important aspect about J'onn is his voice. Manhunter isn't a physical character, and that's not his main form of expression... when J'onn speaks telepathically, it should be powerful. I think Dennis Haysbert is one of the best choices in here, though any listed so far would be great.
I also think a costume, of the quality and style of Hellboy or Thing in the movies would best fit such a personable character... of course CGI will be involved when J'onn flies or morphs, however I think him being there as a 'person' and not just a construct will go a LONG way towards making him relatable... tangible... real.