I voted for Honda because of the overall scope and their reliability, quality, various innovations of the past and their racing history.
But they all have a claim to being the best in some ways.
Aprilia is putting out some very impressive bikes nowadays.
Ducati has put racing twins on the map. Of course that has been done with way overpriced bikes of lessor quality and reliabilty. Can't argue with their successes though. Sound familiar Harley? Yes they are the Harleys of the sportbike world.
Yamaha made the open class bikes a viable market and has almost single handedly destroyed the 750cc sportbike market in the process. They also put a little spark back into the 600 class. Got spanked again quickly but still deserve alot of credit.
Suzuki has the greatest linup of sportbikes on the road currently IMO(GSXR600/750/1000, SV650, Bandits, Katanas, TLR/S, Busas). Now if they could just get their reliabilty up to par with the others and not screw over their customers when things do go wrong. That issue is the only thing that keeps them out of the top spot I think.
Kawasaki just can't hang right now. They have done well in the past but are sucking hind tit now. All I can say is they are quite reliable and are pretty damn good street bikes. They will rise again someday I think.
Having said all of that, Honda is the best. They cover it all but lets face it... they are all pretty damn good... even Ducati.