The evaluation of music's artistic merits is highly subjective, so there's bound to be some disagreement in this attempt. Personally, I find that each soundtrack has its successful pieces, and its less effective ones. As a result, I prefer not to endorse one entire soundtrack over another, but rather comment on the relative strengths of each one, since there is much that is good in each of them.
The Witcher's soundtrack seems the most 'cinematic' and thematic to me: Each piece evokes a clear story of the event or location it accompanies. For better or worse, the game's main theme is readily discernible in many of the pieces, and, while there may be a lack of variety, is memorable, and suitably fitted to the actions of the game. (Some of my favourites are in this one.) It seems the most 'unified' soundtrack of the three, with only a few outlying pieces.
Assassins of Kings has some highly effective, varied, and often quite pleasant, pieces, but -- with some exceptions -- is limited to what I consider 'background music'. While a great deal more complex instrumentally, when compared with the first game, not much really stands out, nor over-powers the imagination; but rather works well as an accompaniment to the various settings, making suggestions of the environments and events. There is a certain grandeur in some of the tracks, relative to the increased scope of the set-pieces. There is one instrumental choice, however, which is at odds with the game's setting: electric guitar, which is used in some of the action pieces. This takes the music outside of the game's world, and makes it rather more 'generic' (it could be used equally well in another game, not set in a mediaeval world).
Wild Hunt receives commendation for the range and number of pieces (in-game), however, save for a few memorable themes, not much seems to leap out while playing. Hardly ever do I find myself stopping while adventuring to admire the music, whereas in the previous games, I do this frequently. Generally speaking, the music has a stronger Slavic character, and the vigorous performances by the Polish folk-metal band Percival tend to characterise the title's sound. Vocals are more apparent than in the previous games, and actual songs make their appearance for the first time. This soundtrack is perhaps the most 'unique' in relation to the others, and is thus difficult to uniformly assess, due to the variety of music to accompany its multiple settings. (However, I do find it's sometimes easier to listen to this one separately from the game, in order to appreciate it.)
