When I see Yuyuko's Overdrive, I see it almost like the arms of a galaxy spiraling outward. Yeah, I'm a geek like that. :V
As for best visuals, I think MoF takes the cake here. ZUN put a lot of effort into the Spellcard designs, something he hasn't truly been able to match since. The stage backgrounds are superb, especially Stage 3, and they're all accentuated with the floating leaves everywhere. Autumn really is the most beautiful season if you ask me, and that certainly helps the visuals for a game that specifically takes place in that time of year.