This is the only type worth buying. Single-wall tumblers sweat and lose temperature quickly. Look for 18/8 stainless steel construction.
20 oz for a single drink, 30 oz for all-day use, 40 oz for those who hate refilling.
Sliding or push-button lids are most convenient. Screw-top lids are the most leak-proof but slower to open.
The YETI Rambler 30oz remains the benchmark for insulated tumblers.