Genre: do whatever the hell is easiest for you; there are 2,405,518,376 ways to tag genre in music and that's roughly the same amount of internet users in the world.
I wasn't aware that ARRANGER was a tag.
Unless you mean ARTIST? Regardless, I put whoever is directly responsible for the production of the music you hear. ie. Hiro Tanaka composes the Samus Appearance Fanfare jingle, which is used in Super Metroid, so in
Super Metroid "Sound in Action" track 30 is credited to 'Hirokazu Tanaka' for COMPOSER and whoever arranged it for Super Metroid is credited as ARTIST, however, that track is uncredited, so... screw [pun intended] us, right?
If there are vocals involved, I like to credit the singer in the ARTIST field as well and credit the lyricist (the one who wrote the lyrics) in the COMPOSER field as well.
For ALBUM ARTIST, if there's one artist the stands more prominently than the rest, I just use that. If not, I'll sometimes pair two or three artists on that field and even make a distinct artist folder for them. If there's anymore than that, I'll just type in 'Various Artists' and file the album under a 'Various Artists' folder (eff your 'Compilations,' iTunes!).
Take
DK Jamz for example. Two prominent composers: David Wise and Eveline Fischer (or Eveline Novakovic now). But there's also one Robin Beanland track and an arranged track by Yoshiyuki Ito. So I use 'Various Artists' for the ALBUM ARTIST field.