I haven't completely made up my mind about 2018 but this is a first list. Some major releases from the last quarter of 2018 already considered, so it may still take a couple of listens to change the ranking.
Best Game Soundtrack:The major releases did not disappoint. Octopath Traveler met all expectations and Xenoblade 2 deserves a honorable mention for the effort
Kurokishi to Shiro no Maou at 4 discs turned out thoroughly enjoyable.
Best Eroge/Visual Novel Soundtrack:A miserable year for the eroge industry and it shows. Visual Art's is strong as ever with their Key and SAGAPLANETS brands, but I was sorely disappointed by the monthly release output. Paradise Ocean was OK and Raspberry Cube pretty good, that's it.
More Music Entertainment had a productive year but I just can't get into their works. It also feels like artists such as solfa or Toshinori Orikura get all the jobs now and are doing their best to drain this genre of any innovation.
Best Western Soundtrack:Celeste outshined everything in the indie department in 2018. Moss delivered a great fantasy score and while I typically don't care for horror game music at all, Chad Seiter's Arkhangel works so well out of the game, it deserves a honorable mention.
Best Arrange Album:Too many orchestra CDs this year, it used to be something special, now it takes 5CD Suikoden concerts or Eroge Chamber Orchestra CDs to even take note. I'm happy that Key Orchestra takes a completely different path to AUGUST's Traveling August concert series with focus on BGM rather than vocal songs.