I don"t have a favourite sandwich. I do have a favourite sandwich for different parts of the day.
QFT.
Further than that, I think the texture & flavour has to be right.
You want some crunch, some softness, some bite, some tang...all in one.
E.g.
Toasted Bacon Sandwich with HP
Mature Cheddar & Large Onion Slice in a doorstep.
etc. etc.