For me, it's the 1921/28 Thompson SMG. Why? I like the finish on the gun. I like the round it shoots. I like the history behind it. And I love the sound of it when it's rocking in full auto! That gun just says, to paraphrase ZZ Top, "I'm bad. I'm nationwide!"
Other than that, my favorite is my Glock 30. Why? It saved my life.....
And then after all of that it's between a Para Ordnance 18*9 and a S&W 649.......