Kaka is the gold standard of all you can eat sushi in Markham.
It is expensive and you must make reservations in advance, but it's still a favourite for most people. The food and service is great here. They're most known for being one of the first restaurants to offer pressed and torched sushi on an all you can eat menu, which is now starting to become increasingly common. They are located in the First Markham Place plaza, so parking can be a bit of a hassle. The restaurant itself is small but nicely decorated and clean. During dinner hours (even on weekdays) you have to make a reservation or else you'll be waiting for an hour with no guarantee of getting a spot. Additionally, they will actually enforce the seating limit since the demand is so high.
The food quality is great here. On top of their aburi sushi, what also sets them apart is their extensive dessert menu. They serve creme brulee (original and green tea), as well as little mango slushies in shot glasses.
You probably aren't getting the best bang for your buck, but if price isn't an issue at all, go here.