Chipotle.
For 3 reasons only
1. It's much healthier than Mcdonalds or Burger King.
2. IMO best fast food in terms of quality and. quantity.
3. My late Grandmother often ate at Chipotle when she visited America, and so eat there in tribute to her.
The only bad thing is that there is only 2 of the places in the Entire UK, both in London.
Second favourite would be Burger King.