Cheaper sushi restaurants serve a product called 'white tuna' which is not actually tuna, but another heavily oily fish which is not recommended for consumption as it can cause excessive loose stools. If you see something called 'white tuna', just avoid it.

Albacore and yellowtail are the 'white flesh' tuna versions - yellowtail/hamachi is very delicious, while albacore is harder to find raw (and usually the stuff that ends up in cans) - but if you can find it raw, it's delicate and delicious.

I would steer clear of bluefin and other red-flesh tunas which are being over-fished to the point of extinction.