I had this exact conversation on facebook a few years ago and it started a great debate. My only rule was you can't include specific versions of a sandwich you really like at one place. The merit of your vote would be judged by the conventional ingredients/construction of the sandwich. So you wouldn't say "the italian at Home Slice" but "the italian sandwich". I ultimately landed on 1) BLT 2) Reuben 3) Italian 4) patty melt (because a cheeseburger isn't a sandwich but a patty melt is and I can't tell you why that contradiction exists, but it's a fact) The simplicity of the BLT wins every time. And Reubens are just typically fucking awesome.
Now, I might like an Italian Beef sandwich from any number of places in Chicago more than a BLT from Jersey Mike's. But Italian Beefs are regional and not typically executed correctly at most places outside of Chicago. And the Chilanga torta at Mi Tradicion might be better than a Reuben from Bella Donna subs. But still, the Reuben is a superior sandwich than a torta.
Edit: If it were more widely-available, I think the jambon-beurre would make my top 5.