If it vibrates, while idling, like a Harley, I walk away. I ride in-line four's for good reason: they don't shake when idling.
Other than that, I prefer the CB750F at the bottom, but I like older bikes, particularly of that style. V-twins just aren't my style, no matter who makes them, no matter how smooth they may be. YMMV. Cheers!
1979 CB750K (sold, 2012, but not forgotten)
1983 Kawasaki 440 LTD Belt Drive (sold, 2011)
1993 Kawasaki Voyager XII