what is the best brand for cod liver oil?

I think they intended for you to look for fish/cod liver oils from wild caught fish sources. The rationale behind this is that these fish are higher in EPA & DHA than their farm raised counterparts due to their natural water vegetation diet, as opposed to farm raised fish which are often grain fed.

Edit: Yes cod liver oil is good for your joints. It is also generally a good source of Vitamin D unlike most other fish oils.