My offhand guess would be KinKi, just because they've been around a while and since there's only two of them, there's none of that mix-and-matching you get with groups. But I suppose it would depend a lot on whether you see any picture where they're remotely in the vicinity of each other or smiling at each other or anything like that as shippy. (Therefore, I deem this question impossible to answer without major bias. XD)