I have to say that I LOVE any sound that he makes :)
But to answer your question, if I HAD to pick one accent - it would be his regular speaking accent. The one he uses in interviews :) He has said that he has never used his own "real" accent in any movie he has made. But whether he is speaking, singing, laughing, sighing, or making any sound effect really... he always makes me smile - or melt - whichever the case may be :)
For me i like both accents he sound great in his british accent and i Love it,and also he sound good in his american accent,for me i like both because i have my puertorrican accent and if yoy know a puetorrican you know how we talk. Im trying to speak and write well my american accent.Still i love Robert in american or british!!!