Best 660 exhaust??

I have a 05 660 what's the best exhaust for it? Single or dual?? Muzzy?

Depends. You care about how loud it is? Alba seems to be pretty sweet especially since it doesn't come with the maintenance of muzzys. And at cruising and idle it is just above stock loudness and at half throttle it's just below 100 dbs. The muzzys also make good power but they have to be repacked and screens cleaned. But with whisper cores they are nice and quiet. I drilled mine out like James said to do and they rock. Quiet how I like so I'll put up with the maintenance for now. Plus the power is great over stock. There are ones louder out there that make even better power as well. But I would say go with a full system over slip on mufflers. 

I would also say try grinding the welds in your stock pipe and a bigger tip too. That might be as much as you're looking for.