The 'favourite sandwich' question must be one of the most frequently asked here on AB!
My usual choice of sandwich in a sandwich bar (when they've got the ingredients) is Stilton cheese and mango chutney, served on granary bread. (A sandwich shop in Ipswich doesn't have Stilton but they do have Danish Blue. Somewhat surprisingly to me, that works quite well with mango chutney too).
A favourite of mine at an auction room café I occasionally visit is cottage cheese, peanut butter and celery. (Sounds weird; tastes delicious!)
However there's still nothing that can beat a good old cheese & chip butty ;-)
As boys, my brothers and I insisted on Marmite sandwiches, whenever we went anywhere. Sometimes Mum used to put some crispy Cos lettuce in as well, which was really nice. Its getting difficult to find Cos lettuce in the shops these days....its all this tasteless Iceberg rubbish.
Many years ago, when I was backpacking around Europe, I first tried peanut butter, mixed with jam, that an American student gave me and its still one of the favourite.
Preference would be for what my dad always had, a bap/balm cake/roll with Spanish onion and cheddar cheese topped with home made piccalilli -fabulous. just seems a shame not being able to get the giant sweet onions any more:-((