BEST is definitly the absolute wrong word but anyways... To the untrained eyes, and unless you're comparing models within the brands, both Canon and Nikon would look and produce pictures pretty much the same and is the price that will justify the purchase.
To many professionals, Nikon produces a more natural colour rendition and somehow usually towards the yellowish tinge (in a good way) whereas Canon produces a more saturated and rich colour, something which I prefer but to some, it's too fake. So.... there ya go.
Before you're further slammed here (or in any other shutterbug alike forums for that matter) LOL, I suggest doing yourself a favour by googling more first.
If you want to know which one is best, try to know yourself best first. You should find out what you need and what you want to do with them and decide which one suits you more. Anyway, you can't get everything you want from either one of them. You need to compromise. Some people hates to admit that they have made wrong decision choosing a certain brand and then they would say the brand they are using is very good. If I say which one is best, will you accept? If you know your need well, you will find your best camera. It's not Nikon nor Canon. It's you.
By the way, each of them have particular attributes (such as what "rendition" mentioned). Many people may say Nikon has good built but if you have better feel with Canon, who can stop you?
If one of these two brands is more advanced in every aspect than the other, the other has long gone out of business already. To me, I am just glad that these two are very competitive and we can get our camera with cheaper price since either one of them can't monopolize the market. Brand is important, of course. However, it's more for the company. Do more research on your own. Wish you find the one that best suits you.
Your question should go deeper than this. You should tell us:
(1) what is your experience on taking photos/using what types of cameras
(2) what type of photography you will be using the camera for
(3) what budget do you have
(4) best if you have already done your own research and name 2-3 models for comments
From your one lined question, most people might think you are a troll trying to incite a brand war in the forum.