Don't know about worst, but ugliest to my eye has to be the BMW R1200 CL Montauk. Every time I see one (not often, thank the benevolent diety) I am reminded of the face of a jumping spider upon its approach, and it doesn't get much prettier as it passes.
To think that the very same factory produced the R100RS, what I consider to be one of the best-looking bikes ever.
'7? Benelli 650S, '80 BMW R100 RS, '93 Ducati 900 SS, '07 KTM 990 Adventure