Canon EOS-1D Mark IV
Putting the EOS-1D Mk3's demons behind it Canon has produced an upgrade that's not just better, but delivers an incredibly versatile tool that blurs the 'sports camera/studio camera' line more than ever before. The Nikon D3S might beat it in very low light, but if you want speed and resolution the EOS-1D Mark IV delivers convincingly.
Professional shooters needing fast, high res performance
Pros working in ultra low light - the D3S is still better