This is for a special occasion so we want a very good place. Can anyone recommend one?

Sorry, can't recommend anywhere from my own experiences but here's a link for a list of Japanese restaurant reviews in Bangkok. Note that the search option on the right allows you to choose either 'Japanese' or 'Sushi', and also choose a particular area (Silom for soi Thaniya, although why limit yourself to just that soi?). I find their reviews pretty accurate. Most have an out-of-five star rating which is handy as a general guide so you don't have to read the entire review (there's a lot of restaurants).

Hope this helps.

http://bk.asia-city.com/search/restaurants?cuisine=73&area=All