all 3 of them are just cameras that you stick a lens on the front of. Its what you take photos that counts. A photographer can take an amazing photo on anything from a 50 quid digital point and shoot, right through to a 6000 quid canon 1ds, or even something ridiculous like a digital leaf/Mamiya etc. Camera manufacturers want to kid the amateur into believing that it actually matters what camera/lens you use, because it effects that mythic attribute called 'quality.' But in reality a good pic is a good pic, no matter how much noise/lens flare/distortion there is.
So, which is best? 40d d300 or 5d? To be honest they are all toy cameras made out of plastic and will explode if you drop them. But hey, who cares, use, whatever you want, because in the end they are all do the same thing