Out of those three I'd have to say the classic robot fits the best. There's a might bit of curvature clash between the Pandora and slogan fonts, particularly the o's, one being square the other being circular. But some of the other curves (the n's for instance) match perfectly, so my eyes don't hate it.
The Serious font gives a retro feel but lacks in legibility, plus I'm not sure the retro niche is the way to go here. Although retro-gaming is prevalent with the device, equally prevalent is the stance that the Open Pandora is not exclusively designed for this use. So it doesn't make sense to broadcast that in the slogan.