I would say Nandos is my favourite. Mmmm chicken
Nando's is like the upmarket KFC
Macdonalds although a hit and miss sometimes it feels like I'm chewing cardboard but some stores they get it right and I love the way you can now order at those touch screen self service points and have the food delivered to your seating zone.
Itsu is a great sushi chain, pricey but when you crave the sashimi that's the place for me.
Burger King I crave every now and then but only the restaurant near Aldgate near the whitechapel art gallery
Brick lane is another great place for food especially on the Sunday morning/noon times. Lots of vendors selling food from all corners of the earth in one spot.
Also any shop that does decent middle eastern; Shawarma, falafel and doner kebabs :Q
EDIT:
If i'm not lazy... I make my own fast food.