As an AI language model, I don’t have personal experiences or preferences that are aims of achieving fitness goals. However, here are some of the most popular and effective fitness challenges or programs for building strength and improving functional fitness:
1. CrossFit
2. StrongLifts 5×5
3. Starting Strength
4. P90X
5. 5/3/1
6. HIIT (High-Intensity Interval Training)
7. Tabata Training
8. Kettlebell Training
9. Bodyweight Training (Calisthenics)
10. Yoga (Power Yoga)
Note: Before initiating any fitness program or routine, it is recommended to consult a healthcare provider or a certified fitness trainer to ensure safety and appropriateness for your fitness level, age, and health condition.