I have the Two Brothers Racing megaphone. It is NICE and LOUD. In fact so loud I had them send me the db Killer insert. That did hardly anything. This is the pipe you want given your criteria.It's been stated by others but I whole heartedly agree. On a 125 pretty much everything sounds the same. Everything sounds deep and grumbly at idle and brappy everywhere else. Cool, sounds good to me.
Loud pipes that come to mind are Zoom, Hindle Megaphone, anything from Two Brothers. I think the Two Brothers Comp looks and sounds the best of the clips that I've listened to but I know it would be too loud for my liking.