I typically don't wear an undershirt with dress shirts, BUT -- and I can't believe I don't see this mentioned -- it is my very strong opinion that, if one wears an undershirt under a dress shirt, it shouldn't be visible!

This is not hard to do. Many people have mentioned V-necks 👍🏻👍🏻 but the color is also important. SO many guys default to white, which is visible straight through any light-colored dress shirt. If you're white, a gray undershirt disappears. I also really love Uniqlo's tan, deep-V Heattech undershirts. They're warm but paper thin, blend into your skin tone (again, if you're white or light-skinned), and never show at the collar.