The best exercise for someone in their 40s depends on their individual goals, fitness level, and any specific health considerations. However, generally speaking, a well-rounded fitness routine that includes a mix of cardiovascular exercise, strength training, flexibility work, and balance exercises is ideal.

Cardiovascular Exercise: Activities like brisk walking, jogging, cycling, swimming, or dancing help improve heart health, endurance, and overall fitness. Aim for at least 150 minutes of moderate-intensity aerobic activity per week, or 75 minutes of vigorous-intensity activity, spread throughout the week.
Strength Training: Incorporating resistance training into your routine helps maintain muscle mass, bone density, and metabolism, which can decline with age. Focus on compound exercises like squats, deadlifts, lunges, push-ups, and rows, aiming for 2-3 sessions per week.
Flexibility and Mobility: Stretching exercises help maintain flexibility, improve range of motion, and reduce the risk of injury. Incorporate dynamic stretches before workouts and static stretches after workouts. Activities like yoga or Pilates can also improve flexibility and core strength.
Balance and Stability: As we age, maintaining good balance becomes increasingly important to prevent falls and injuries. Exercises like Tai Chi, yoga, and specific balance drills can help improve balance and stability.
Functional Training: Incorporate exercises that mimic real-life movements and activities to improve everyday functionality. This could include exercises like squats, lunges, and overhead presses with free weights or resistance bands.

It's essential to listen to your body, start slowly if you're new to exercise or returning after a break, and consult with a healthcare provider or fitness professional to tailor a program to your specific needs and abilities. Additionally, remember to include proper warm-up and cool-down routines in your exercise regimen to prevent injuries.