RAW.
All digital cameras do some processing of your photos between the information coming off the sensor, and the file being saved onto the memory card. The processing includes white balance corrections, brightness, contrast and saturation changes and some sharpening, all designed to ensure your pictures come out perfectly every time.
Serious amateurs and professionals sometimes prefer manual control over such processing, so many high-end cameras offer the option to save in 'RAW' format – pictures with virtually no in-camera processing applied.
RAW files cannot be opened by most applications – they need to be converted to JPEG or TIFF on a PC using special software supplied with the camera or third part programmes. This software allows you to change things like white balance presets, sharpening and even exposure before exporting (though most usually have a default 'auto' setting too).
RAW files offer experienced users the best possible quality, but the process is more convoluted, and really requires you to know what you are doing! As a rule of thumb RAW files are usually much larger than JPEGs, but a little smaller than TIFF files.
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OK. So you want the absolute highest quality but you don't want to use the RAW process. In this case after downloading your images as jpegs, save them to a separate folder as uncompressed TIFF images. Then load these images for editing and save them as jpegs when you have finished. If you do this, the original Tiff images will remain at their original quality.