What is the best sandwich filling combination?

Devil Ham a little mayo on whole wheat bread.

Tofu fried crispy in nutritional yeast with mustard, lettuce, tomatoes and pickles on toasted rye..or bbq seitan with coleslaw on wheat buns. Both are amazing. 

bacon and mayonnaise

The "best" it what I have a taste for at the time. 

A great BLT is wonderful in the summer with fresh ripe tomatoes but not the greatest in the winter with hothouse tomatoes. 

Some favorites of mine..

Hard Salami (has to be Volpi Genoa salami (with smoked or regular)Provolone cheese, Roasted red peppers and some hot giardiniera.

or

A Chicago style Italian Beef sandwich, wet with giardiniera 

or, I just saw this in a restaurant, thought it was different and ordered it..very good..

Apple bread, a bit of cranberry mayo, smoked turkey, smoked ham, Gouda cheese grilled until the cheese has melted. 

And I do have to join the Reuben Sandwich group as that is always a good choice as is a good Hot Pastrami.  Both HAVE to be on Rye though

Anything as long as there's bacon in it.

Strong Cheddar Cheese and Branston Pickle

Chicken and Emmental Cheese with Creamed Horseraddish Sauce and Lettuce

Tuna mashed into Mayo and Tomato Ketchup with Roasted Red Peppers and Cornichons

Peanut Butter and Banana .. Fried in Butter !

A Rueben or a BLT

My favorite would be canned crumbled ham, finely diced onion and celery, no salt but a bit of pepper, a tiny squirt of mustard and enough real mayo to make it into a 'salad' consistancy. Depending on who I am making this for I may also add a couple of dashes of hot sauce intead of the mustard, and sometimes I will add a bit of green sweet relish for a change up.

Any kind of Meat and Cheese. 

Several things with salad, cheese, ham, tuna and mayo, egg mayo... It's the salad that's more important, no iceberg lettuce because it doesn't taste of anything, it has to be a peppery or a bistro leaf mix, with added tomatoes, onions, peppers and pickles. If it's a sandwich for work I keep the salad and any dressing separate so the bread doesn't go soggy. Oh and, a tiniest pinch of salt into the salad mix.