The best that I have found,
sushi=Ru Sans, Virago
steakhouse=no great niche steakhouses. Nashville has Palm, Ruth's Chris, Morton's. I've never eaten at
Nick and Rudy's, which is a steakhouse, but it has received mixed reviews.
Italian=
Mario's is a really nice place downtown.
Valentino's is another nice place in the downtown area, but I've never been there.
Mexican=From what I've found, Cozymel's is about the highest quality Mexican food in or around Nashville. There's a great TexMex place called San Antonio Taco Company(Satco). They are known for their queso and fajitas. I love it and go there at every opportunity. Satco has a large deck and serves beer, so it's a nice place to hang out when the weather is nice. Las Palmas is a straight-forward mexican restaurant where most everything is under $10. It's nothing exceptional, but the food is good and it's always busy, esp. at lunchtime.
Cup