On paper, Bisping wuld look like the strongest candidate. However, you also have to take into account the extent to which his UFC career may have been planned and managed by Zuffa to promote the UK market. Maybe not that much has gone in that regard; maybe it has and even it has maybe he would have as good a record anyway. Nevertheless the doubt is there.
Therefore on the basis of a better win-lose ratio to date, I'd have to say Hathaway
PS I know there is an argument against weighting the UFC too heavily but at the end of the day it's the closest thing MMA has to a bona fide world championships