Mine favourite changes with my mood, what's usually at the top of the list though is a fried egg sarnie with brown sauce.
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BROWN sauce? Heathen!
At this moment in time, the current favourite is Cheshire cheese, celery and mayo on a wholemeal bagel.
Don't think I have one favourite, but the salmon & cream cheese one vies for position. A good crumbed ham & tomato works too. As do the various mayo based fillings such as coronation chicken or tuna & sweetcorn, etc. A decent cheddar is good too. Or a steak & mustard or fired onion sandwich, that's mouth watering. As is bacon & fried egg.