When dealing with undershirts, it is important to understand how the shirt going over the undershirt is being worn. For most men there are three options, with a tie, one button undone and two buttons undone. To best accommodate each style of shirt there are a few different style of dress shirts. To start off lets look at the most common solution, and the one I prefer.
Gray V-Neck Undershirt -
This is the most versatile undershirt in your closet and will work for most guys with most outfits. A gray undershirt is superior to a white undershirt since it will not show through from under a white dress shirt. The gray undershirt provides no visible undershirt lines (the male version of pantie lines). Since gray will not show through any light colored shirt, it makes it the most versatile color in the wardrobe. As a V-neck, the shirt allows for the top button to be undone without showing a t-shirt underneath. If the shirt has a deep enough V-neck, two buttons can be undone with a shirt being seen.
When I want a more casual look I appreciate the layered look of showing a gray v-neck shirt from under a button-down shirt. If the button-down shirt has two or three buttons undone and the gray shirt underneath, the look can be quite cool, it is a very sprezzatura and nonchalant look.
For those that do not like to have a shirt showing underneath, ever, the best option is a deep V-neck undershirt. Depending on the amount of buttons that you typically have unbuttoned, the best option I have found is the Alternative Apparel V-neck shirts. The Eco Boss V-neck is a deeper than normal V-neck and comes in two great heather gray colors. It is also extremely comfortable cotton and is nice enough to be worn all by itself as a shirt. The Alternative Apparel Eco Boss runs for about $28 direct or for only $15 thru Urban Outfitters.
For the people that demand an even deeper V-neck shirt I recommend the Alternative Apparel deep V-neck burnout shirt. It comes in a few extra washed colors and fabrics. It is a very soft cotton fabric and runs for about $30-$40, and featured below.
Lastly, for those that don't care about the depth of the V-neck and do not want to pay so much for an undershirt, Gap is a great option. The Gap sells two heather gray shirts for under $20, these shirts are quite comfortable and fit well. I also recommend the Uniqlo undershirt. It runs for about $10 per shirt and has fuller body than the Gap shirt.
Depending on your preference, think about gray... it makes for a great versatile alternative to white.