Everyone had their own tastes. These are mine:
A good sandwich consists of a high quality rye bread - "Jewish rye" without seeds, preferably from a small bakery - not commercial rye bread such as Pepperage Farms, Arnold, etc. That's too soft and to me seems like white bread with caramel coloring added. The exceptions to this are "sweet sandwiches" such as peanut butter, jelly and a combination of the two. For that I prefer a hearty white bread.
The spread depends on the filling. Mayonnaise is almost always used. I prefer Hellmans (AKA Best Foods) but other brands may be acceptable. Sugary spreads such as Miracle Whip should never be used.
Mustard may be used with some fillings such as ham and liverwurst. It is there for flavoring not to add moisture. That's the job of mayonnaise. I prefer Guldens mustard but also like Grey Poupon. I dislike sugary mustards such as honey mustard.
Cheese. I don't care for processed cheeses such as "American" cheese. I prefer "pure" cheeses. Which one depends on the meat. Ham gets Swiss, salami gets provolone, liverwurst gets slicing cheddar or perhaps Asiago.
When in season, I will put thin sliced heirloom tomatoes on the sandwich. I'm very picky about tomatoes. Most store tomatoes are absolute junk. I either grow my own or buy "specialty tomatoes".
A small amount of lettuce may grace my sandwich, but it depends on the filling.
I never put pickles on the sandwich - such as "pickle chips." the pickle is to be eaten "on the side" taking alternate bites of the sandwich and the pickle. I very much prefer "barrel pickles" which are made by fermentation - not "jar pickles" which are made in a vinegar brine.
Other ingredients may include finely chopped celery in a tuna sandwich and an egg salad sandwich.
Of course, subs, hoagys, grinders, etc.; the Italian sandwich made with hams, onions, cheeses, shredded lettuce olive oil and herbs are another thing entirely and their proper preparation would take another five paragraphs.
Also, I make my own sandwiches unless I buy one when away from home. I'll probably eat a sandwich four or five days a week when doing road work. If at home during lunch time, I may or may not eat a sandwich.