A.O.I. (outlets in The Emporium and Silom Soi 6) has good quality Japanese at reasonable prices, for Japanese food. Problem is that the decsriptions of sushi are not that helpful; some rolls were standard, others massive with tons of delicious ingredients and no way to tell from the menu. There's also a soi off Silom that caters to Japanese clientele and is popularly known as Soi Japan. Japanese travellers don't partronize bad places long, so I'd venture to guess that most of those spots are good quality. Though I haven't experienced it myself, Shintaro at the Four Seasons is reputed to be the best Japanese in town.

You didn't ask about Thai, but I had such a good meal at such a great price I'll recommend Baan Kanitha anyway. The branch I went to was on or near Wireless Road in the Embassy district, and it was excellent, very authentic, very reasonably priced.

Mei Jiang at the Peninsula has excellent Chinese with stellar service. Leans toward Cantonese but also had a wonderful Peking Duck. Worth going out of your way to get there.