What is the best extender?

It depends on your needs and what works the best for the stud dog.

Minitube Chill 10

Depends - I've had good luck with Chill 5. If you own the stud then test with Chill 5, 10 and Clone and see what works best for him. Some dogs do better with one than the other.

Camelot Farms is excellent and I have also had good results with Kenney.