What is the Best ......

On the cheap side, Aussie 3 Minute Miracle is great for a deep conditioner, and they have a nice leave-in as well. But I definitely agree with the Biosilk rec!
 
Plain old coconut oil. It has worked just fine for billions of people for thousands of years.

Just put a tiny bit and work it through from roots to ends, don't use too much, you are not trying to look like Vajpayee.