Since Toei uses Den-O a lot and he's popular I'll say Den-O. But Kuuga was the begining of the Hensei Riders so he gets credit as the face of the current generation too.

Someone not showering Kuuga with praise every chance they get? Heresy!

Really though, I'd say whoever the newest guy is at the time is the face for the year, and at that, aren't Rider #1/2 the faces of the whole franchise? They show up everywhere and get more merch than any Rider who's show has ended.

Seriously, I'm not even talking about the anniversary-related stuff, they appear in stage shows that otherwise have nothing to do with them.