AYCE Sushi on Yonge St between Dundas and Bloor
I think that you will be hard-pressed to find a "good" AYCE place on Yonge street, but if you're looking just for any basic AYCE within that stretch of Yonge, here's a brief list:
Natural Japanese Sushi - 714 Yonge
Spring Rolls - 693 Yonge
Takumi Sushi Restaurant - 484 Yonge
Spring Sushi - right off Yonge in the old Woo Restaurant - 10 Dundas St. East
Oh my god NOOOO! the Springsushi (one word) at Yonge Dundas is the place to BE, not to EAT. Their gimmick is the menu is on a tablet, you scroll through it and check off the items and give the tablet to the server who will send in the order. I had a bento box which had sushi, and had to ask for wasabi and ginger. The rice in the sushi was hardly seasoned. I couldn't finish the grilled salmon so asked for it to go. The server said did you want to take that (indicating the leftover tempura) too? I said no. The takeout box never appeared and when I asked they found the food was thrown out! They wanted to give me a $10 voucher, but after waiting for 10 more minutes I decided I don't want to go back anyways so we left.
Natural Sushi is a place I wouldn't go back to either, though last time I want was years ago.