They are all good! Mix it up and get good results. In general I find low reps (6-8) work best on compound movements and 12-15 on isolation movements for me.
Some ways to mix it up:
1. pyramid (reverse pyramid) 15,12,10,8,6 adding weight each set, or 6,8,10,12,15 removing weight each set
2. start your workout with a compound movement for 4x6-8 reps, then do an isolation movement for 2x12-15
3. do varying reps on alternate workouts 12-15 one workout and 6-8 the next time you do that same body part.
4. do cycles, where you start with a weight you can do 15 reps with easily and then add 5 pounds every workout uitil you can't do 6 reps anymore and then start over with the same 15 rep wieght again