My #1 would have to be one of our burgers, made with 1/3 lb. good ground chuck, seasoned with S&P only, and cooked just until the juices run clear. I like mine on a toasted bun with mustard and relish, and topped with cheese and a garden fresh tomato. If I don't have a good tomato, I add a little squirt of catsup. I also enjoy a few grilled or caramelized onions upon occasion, but no raw onion, thank you.
I'd just as soon have a really good burger as a steak.
Very close would be a good smoked or Polish sausage, preferably grilled, on a toasted hoagie with mustard, kraut, and Mozerella or similar cheese. Right now, I have some wonderful homemade picalilli, made by the ladies at our church, and I like that even better than the kraut.
Then there's the BLT...when tomatoes are in season, we have them often. Sweet succulent tomatoes, still warm from the garden, smokey thick-sliced bacon, a tender lettuce like Buttercrunch, on toasted bread spread with Miracle Whip, tastes so good I tend to levitate. For a variation, I sometimes make a club sandwich out of the BLT by adding a few slices of grilled chicken breast and whatever cheese I have on hand.
Those are my 3 faves...sorry, I couldn't pick just one.