Another thumbs up for the Smedley polos. They truly are the softest polos you will find. I'm a 44 chest 34 waist and the L is pretty tapered on me. The arms are fairly tight however. Their customer service is also very good, I've emailed them a couple times and have always gotten really quick responses. I had to return an item and received my refund in 5 days, including shipping time from the states to England! They also remove the VAT for US customers although they add a steep delivery charge.
I bought a Cucinelli polo from Lance, the cotton isn't quite as nice but it is even more tapered than the Smedley polos and the construction is pretty good.