Basic panel

WB : As shot
Ignor exposure, tone settings as they will use AUTO by default.

Tone curve

Keep it neutral

Optionally add a vignette

Save as your default LOOK.

Be sure to check ALL SETTINGS and check APPLY AUTO TONE.

This preset can then applied to all images as the first step.

IF you want these defaults to automatically apply to all your RAW files...

You have to set up the Camera RAW preferences to Apply Auto Tone Adjustments.

The downside to this is that this preference applies to ALL images that Bridge views, in other words, all JPGS, PSD files, TIFFs, etc. will be auto tone adjusted when they display in bridge. This is usually not desireable as the PSD files are probably already adjusted just right in Photoshop and should not be modified again.

This is a major oversight on Adobe's part, in my opinion, as the Auto Tone should be able to be saved as a default per file type, not just globally.

IF saving the settings as part of a default, continue...

And save it. This will save all settings for this particular camera and file type. Except for the Auto Tone part, which is dependent on the preferences and will change when the preference changes, regardless of whether it was on or off when you saved the default.