I think the best cardio choice really depends on the person's goals, fitness level, and what they enjoy enough to stay consistent with.
The stair climber is popular because it challenges the legs and gets the heart rate up quickly, while the treadmill (especially walking at an incline) can be a great option for building endurance with less impact on the joints.
For most people, cardio after strength training doesn't need to be extreme. Even 15-30 minutes at a manageable intensity can provide great benefits. The key is choosing a method you can recover from and continue doing consistently.
I'm curious- do you prefer cardio for improving endurance, burning extra calories, or just supporting overall health?