Unfortunately, I don't know a lot of good restaurants downtown. I tend to avoid downtown because of the traffic and congestion. However, there are a lot of good food carts downtown, including the famous Japa-dog. Best thing is to just explore downtown.
For Japanese, there is a sushi restaurant on just about every corner. Most are good, because of the competition, bad restaurants don't last. Whenever I go to court downtown, I usually have lunch at the Kimono restaurant on Smithe and Hornby. They have decent food. If you are willing to travel outside of downtown, there is also a small restaurant on Oak and 21st Avenue called Tokiwa that have some interesting "Creation" rolls. However, this place fills up fast during lunch time. You can order to go.
For Chinese, my two favourite restaurants are Kirin and Szechuan Chongqing. There are several Kirin restaurants, including one downtown at 1172 Alberni Street. Szechuan Chongqing is on West Broadway, one block west of Granville Street (1668 W. Broadway). Or, you can take the Canada Line skytrain all the way to Richmond, which is essentially an entire chinese city. There, you can pretty much take your pick.
For Seafood, there is Monk McQueens on False Creek, near the Cambie Street Bridge. I also like the Boathouse Restuarant. They have several locations, including one near Stanley Park.
If you like East Indian food, I highly recommend Vij's Restaurant on Granville and 11th Avenue. Be warned, they don't take reservations and they always have a line-up to get in. Their lamb chops are to die for. Other restaurants I like include the Banana Leaf which is a Malaysian restaurant. They are on the 800 block of West Broadway. Their seafood pasta with spicy tumeric sauce is fabulous, and if it is in season, I like their mango sticky rice desert.
If you are planning on using public transit, go to
www.translink.ca for trip planning.
Oh yeah, if you like AYCE (all you can eat) Japanese and Korean food, try the Shabusen. Again there are several locations, including one downtown (755 Burrard Street) and one on Granville and 14th Avenue. You give up quality for quantity, but the korean barbeque is fabulous.
Another thought: I am assuming you are looking for authentic Chinese food, not "American" Chinese food. I lived in Albuquerque for a year, and the so-called "authentic Chinese restaurants" there served some incredible abominations. Seriously, you call that Chow Mein?