The glass in your hand changes the way you approach a drink. A wide tumbler gives ice room to move; a narrower glass invites you to bring your nose closer. Neither shape is a universal answer. Start with what you like to pour.
For whiskey served neat
A tulip-shaped glass has a rounded bowl and a narrower opening. It is a familiar choice for tasting because it gives you room to swirl while bringing the aromas toward your nose. Glencairn describes that as the purpose of its own tapered design. A smaller pour also leaves space above the whiskey.
For whiskey on the rocks
Look for a low, wide tumbler with enough room for your preferred ice. Check the inside diameter, not just the stated capacity: a large sphere has to pass through the rim. A solid base can feel satisfying in the hand, though heavier glassware is not automatically better.
For cocktails and hosting
A rocks glass is a useful home-bar staple for an Old Fashioned or a short mixed drink. Think about how the glass feels at the rim, how easily it can be washed, and where it will live when it is not in use.
Choose for your own habits. A glass you enjoy picking up will see more use than one bought for an occasion that never comes.