I'm going to have to go with Hunter x Hunter 1999 for strongest overall OST. "Ohayo" is a great opening theme, and then both Pale Ale and The Sun Also Shines at Night are really unusual and distinctive openings. Plus the instrumental OSTs overall are really distinctive. A lot of anime, when I hear the bgm, I can't place it or it doesn't really strike me as specifically 'that series'. But the old HxH anime is completely recognizable. I've been watching the new HxH anime this week and while it does a lot of things well, the music overall is totally generic. The action bgm track could be from pirates of the caribbean lol.