The ultimate shirt for spring and summer---time and time again---is the polo shirt. It fits perfectly between dressy and casual and can be paired with a jacket or swim trunks effortlessly, making the thing an outrageously versatile purchase. Here's a few favorites.
Kent Wang Rugby: A hybrid of a casual rugby shirt and a traditional polo, this long-sleeve number is pure style. It also features a button cuff for a polished and cool look that separates itself from the run-of-the-mill alternatives. Can't recommend this one more, just make sure to size up---these babies run small.
Doriani Fine-Knit Cotton Polo Shirt: This knit polo shirt will make you look and feel like an international playboy. Just make sure your stomach is flat or this fit won't be the most flattering thing. Pair with light chinos and brown loafers for a can't lose spring and summer look.
AMI Heartbreaker Polo: AMI is doing some of the coolest stuff in menswear right now and this polo is a look that takes a traditional fit and style and throws a bit of personality on it with the large heart patch. Love it.
Orlebar Brown Ridley Linen Shirt: Perfect for an ultra hot day, vacation life, or a poolside afternoon, this shirt from Orlebar Brown is a mix between a rugby, a linen shirt, and a long sleeve polo. The combo just sings.
Band Of Outsiders Split Pocket Polo: This is a fun alternative polo with a 2-color patch pocket. It has a real nautical-inspired style to it without making you look like a Vineyard Vines catalog model.
Sunspel Riviera Polo: This is the exact polo shirt Daniel Craig wore in Casino Royale, so it's obviously a look worth stealing.
Maison Kitsuné Polo Shirt: A classic look from an awesome French brand. The little red/white/blue fox logo is a nice alternative in a world swimming with polo players and crocodiles.