The best sandwich combination of meat and cheese is Boar's Head ham and Boar's head white American cheese. Condiments, such as lettuce, tomatoes, etc, only add to this American classic.
Using only meat and cheese on a sandwich is boring so you must always try and compliment your meat and cheese. Like for instance putting a little bit of jam with the cheese or a nice mustard with the meat. Ultimately it come's down to your own taste, so why not experiment a bit ?