Hi Givara,
All the signal path settings (Filter, Mapper and Upsampling) result in small changes to the “flavor” of the resulting sound. You will have to believe me when I say that if one were right or “more right” than the others we really wouldn’t offer a choice. However as well as accommodating personal preference they also allow a small amount of tuning for different types of music and different downstream systems.
In particular the Filters all represent different compromises between frequency domain (image rejection) and time domain behavior which can mean that some work better with certain types of music, as described in the manuals:
That said IMO, even for an expert, trail and error is really the only way to determine what sounds ‘best’ in any given situation (although rest assured Filter 1 is a great all-rounder).
Fwiw my own listening (for work and pleasure) is now dominated by streaming, so most of the sources are PCM files (ca 90% std res, 10% hi res) and I tend to keep them that way so I default to DXD upsampling. My personal go-tos for these use cases are Filter 1 & Map 3, but as I said I can find that Map 1 works better for me in certain scenarios. But plenty of folks here default to Map 1 as well. On the rare occasions I am playing a DSD source I tend to keep it as DSD and default to Filter 5.