The best thing to do is workout really hard and get in great shape-- then you will want to go undershirtless. "Crossfit":"http://www.crossfit.com"
Personally, I like the classic look and style of an A-Shirt showing through a good quality dress shirt. You could always look into white dress shirts made of a thicker fabric or knit. http://www.brooksbrothers.com/IWCatProductPage.process?Merchant_Id=1&Section_Id=663&Product_Id=500031&Parent_Id=662&default_color=White#
If you're wearing dark pants, choose a shirt to match (i.e. navy blue or dark grey). If you're wearing light pants, you could go with a long sleeve white tee, which gives your shirt greater opacity all the way down the sleeve.
A grey v-neck t-shirt is the best way to go. The v-neck ensures that you won't see it peeking out from under your shirt, and the fact that it has sleeves will prevent any sweat from showing through on your dress shirt, which a tank top or A-shirt can't prevent. The reason for the grey color is that the grey doesn't reflect light as much as a white shirt does (which is what makes it show through) yet isn't dark enough to show through your white dress shirt. Provided your undershirt isn't so baggy that it ruins the fit of your shirt, no one will even know you're wearing it.
I think the previous answers missed the point. He is wearing a tie so the collar will be bottoned up, so there is no risk of the t-shirt peeking out of the collar. Therefore there is no point in suggesting a v-neck t-shirt. Moreoever, a v-neck t-shirt is more likely to be spotted on your chest creating an unpleasant effect.
I personally think that you should never wear a t-shirt under a formal shirt and tie look, it's just unfashionable. If you are really crying for it, my reccomendation is to wear a tight sleevless t-shirt, with normal collar. This way all the edges of the t-shirt will be covered by the sewing of the shirt (collar and sleeve) and the shirt will be much less evident. With regards to the color, go for a white one.
The best thing to do is workout really hard and get in great shape-- then you will want to go undershirtless. "Crossfit":"http://www.crossfit.com"
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I have generally heard that a gray t-shirt does the trick.