If you're a heavy sweater or doing high-output activities like backcountry touring, synthetic polyester is an excellent choice. It's the fastest-drying base layer material available, meaning sweat gets pulled off your skin and evaporates quickly—so you're less likely to get chilled when you stop moving.
Synthetic polyester is also the most durable option and typically the most budget-friendly, making it great if you're hard on your gear or watching your wallet. Many feature grid or channel textures that trap warm air while still breathing well.
The trade-off is odor—polyester holds onto smell faster than merino wool, so plan to wash it after every day on the hill. It can also feel clammy against your skin if you do get soaked, since it doesn't absorb moisture the way wool does. For hot sweaters who don't mind regular laundry, it's a solid pick.