"Best" comes down to method of ingestion - for chronic pain, preferably a tincture or edible for their long-lasting, global effects. You can read more about using cannabis to treat chronic pain here: https://hellomd.com/articles/how-cannabis-helps-reduce-chronic-pain
Unfortunately, there is no best cannabis product for any condition. Everybody's situation is a bit different: pain level; tolerance to pain; tolerance to THC; amenability to psychoactive effects; and type of pain are some of the factors to consider when formulating a treatment plan. However, I can provide you with a rough guideline, along with product recommendations.
For "conventional" (nociceptive) pain issues, such as arthritis, sprains, strains, dislocations, bone fractures, and myofacial pain -- in other words, inflammatory pain -- I most often recommend a high CBD vs THC edible, spray or tincture, such as the 18:1 formulations offered by Care By Design. I have no affiliation with them; however, I like that they: use the whole plant when making their products, rather than isolating only THC and CBD; offer a variety of ratios; offer all routes of administration, other than combustion; clearly label their products in mgs of THC and CBD per dose; and are relatively easy to find.
Products to consider: https://www.cbd.org/products/drops/18-1-drops https://www.cbd.org/products/soft-gels/18-1-soft-gels https://www.cbd.org/products/chocolates/18-1-chocolate If the pain is localized, you can also try a topical cream. I recommend the creams with THC rather than the pure CBD creams. THC containing topical creams will not cause any psychoactivity. http://truefarma.com/product/sweet-releaf-extra-strength-2oz/
Some very high CBD strains to consider for inflammatory pain: ACDC - a 20:1 sativa strain Ringo's Gift - a 25:1 hybrid strain Charlotte's Web - a 30:1 sativa strains
For non-inflammatory pain issues, such as neuropathic pain, migraine headaches, fibromyalgia and psychogenic pain, I tend to recommend -- if amenable to the patient -- products and strains with higher levels of THC, such as
- https://www.cbd.org/products/chocolates/1-1-chocolate - a "whole plant" 2:1 edible
- http://dosist.com/pen-relief/ - a 1:2 vaporizer pen
- http://kivaconfections.com/products/bars/ginger - a 1:1 "whole plant" edible
- http://truefarma.com/product/sweet-releaf-extra-strength-2oz/ - a THC and THCA topical cream.
Some CBD strains to consider: Harlequin - a 5:2 sativa strain Sweet and Sour Widow - a 1:1 indica strain Pennywise - a 1:1 indica strain Cannatonic - a 3:1 hybrid strain
Hope this helps!