A classic waffle thermal, such as Fruit of the Loom's Waffle Thermal Underwear Crew Top, is always a great and usually inexpensive option. If you want something a little warmer, try a merino wool undershirt, such as MERIWOOL Men's Base Layer.
If you're highly active during the winter, you might prefer a moisture wicking undershirt, such as Thermajohn Thermal Shirts or Real Essentials Moisture Wicking Performance Long-Sleeve Shirt.