"Best" depends on several things. How you're built, what you carry, etc...
Like the above post, I wear untucked shirts over wifebeaters. No real science to it.
One thing Ive noticed is some brands (namely Wrangler) tend to shrink a size when you wash them the first time, other brands (Dickies, Faded glory), dont. They way I'm built doesn't help much, I only weigh 150 lbs or so, but dont have much of a neck. My XL Wrangler shirts are almost too small, and my XL Dickeis and Faded Glory shirts are a little too big. I guess I need to start lifting weights so the rest of my upper body can catch up to my traps...