The one I just made was pretty good.
Fresh deli sliced roast beef (just bought it this morning), red onions, mayo (Duke's, I won't use any thing other than Duke's), and swiss cheese on sourdough. In the oven for about ten minutes at 300 degrees. Get it out put romaine lettuce on it and eat. Simple but tasty.
I also make a tasty reuben, which was the plan for dinner. But I forgot to get the most important piece (corned beef) while I was grocery shopping this morning. Roast beef woudl have worked, but it's just not the same.
Sometimes though I'll buy fairly thick sliced turkey breast and throw it on the frying pan with some honey for a few minutes, just long enough for it to get just a bit crispy and blackened along the edges. Then put it on honey wheat bread with swiss, red onions (I love red onions, but my girlfriend can't eat them

), mayo and green leaf lettuce. Sometimes if I'm feeling adventerous I'll put it on rye or pumpernickel, then it's good with a nice pale ale.
For the most part, I'm not a fan of cold sandwiches, unless it's peanut butter and jelly.