I don't know. According to this article in the Japan Times, it may or may not be Arashi. I know little about SMAP beyond Katori Shingo, who I think is a hilarious and talented comedian when he's not singing and dancing-- good enough to host America's long-running Saturday Night Live and blow its castmembers off the stage. And I know even less about Arashi except they're running ahead of the pack in terms of popularity when I ask my kid students who they like these days.
In fact, out of three kids Friday, Arashi got unanimous approval, the first time this has happened as far as I can remember. It seems in the past, Kat-Tun would garner a vote, or someone I'd never heard of at all, someone so mysterious to me, I'd be forced to just smile, nod and say, "Great!" in response because I wouldn't even know how to pronounce the group's name.
When I first moved here, I taught a lot of college students and they were all into Bump of Chicken.
Right now, it seems only one person would list SMAP as her favorite and she graduated high school two years ago and only comes to visit during her holidays, if she has time. We have one young woman who goes to every Glay concert she can with her mom. The high school girls seem to be mostly into Exile and the junior high kids and younger like Arashi. One guy likes Love Psychedelico.
No one's heard of any of the groups I'm into, which include 5678s, OOIOO, Boredoms, Melt-Banana, Spookey, Puffy, Blue Hearts, Shonen Knife, Electric Eel Shock and Asian Kung Fu Generation. We had one young rocker who likes American punk/pop-punk who started to get into Go!Go!7188, another favorite of mine-- but she's one of the very few people I've met in Hamamatsu who had even heard of them.
Everyone else seems to like J-Pop-- Hamasaki Ayumi remains on top, and sometimes people cite Amuro Namie as being "pretty" but don't seem too into her music-- or else hip-hop. Lots of young hip-hop fans in Japan. And every once in a while, someone will mention Southern All Stars.
So my expert advice to SMAP is to stay the course, guys. Especially you, Shingo.