What's the best format for pictures in order to have the most metadata?

One very good option is to use TIFF format as it support EXIF, IPTC. Also you can store the images in uncompressed, lossless compressed, lossy compressed form. You can have multipage files. As benefit TIFF support 8,16,24,32 even more (for some colour space/compression) bits. And it support RGB, CMYK, YCbCr colour spaces.

But be aware usually the size of TIFF is bigger than JPG.

Post camera processing can cram in as much additional meta data as desired, so I assume you are interested in what the camera produces directly.

In general, RAW is going to have everything the camera offers. Conversion to another format isn't going to create more meta data. It's certainly possible that a given manufacturer may only insert selected meta data in another post process format, but I'm not aware of it with one exception. Multi-image processing in-camera, such as Panorama and HDR may include additional information relative to the additional processing.

WebP has some nice features, but none of those apply to your stated purpose of computing MTF. You already stated lossy compression is undesired, and transparency support would not be used. Most importantly, the output is 8-bits per color channel, loosing dynamic range available in the raw.

For out of camera MTF computation, you need to process the RAW or a format that handles extended dynamic range such as TIFF-16/32/float. In either case you would want to keep the original RAW and its meta data for potential re-processing.

If you want to avoid compression then raw files are your best choice when available.

A camera recorded tiff isn't any better than a jpeg... the image is still processed and data lost/converted in the same way as an 8bit color jpeg; only the resulting image data isn't compressed which makes the files even larger than raw files.

I would use jpegs with very low processing settings (unaggressive picture style) if raw is not available.

Both raw and jpeg formats have extensive metadata support as written by the camera.

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*even though the tiff format is 16bit and could store uncompressed raw data, that is not how cameras utilize the format. Every camera I know of that records tiff files uses 8bit color conversion and processes the image according to the camera settings (picture styles). And, because the tiff image is uncompressed RGB data for every pixel (24bit/pixel), the files are huge compared to 12 or 14bit raw file data (which is why Canon abandoned tiffs).