The good ones are the kits sold at industrial supply places like Grainger's that are made for job injuries. I start with one of these and add a couple quick clotting packets and butterfly bandages and liquid bandage solution and rolls of tape, adhesive and compression. I also carry one of the rolled up leg splints. Duct tape has also been handy on a couple of occasions.
Get one of the flash card sets that quickly cover how to deal with an injury. It will make it faster and easier to respond to an injury.
Think about a broken limb or a serious cut and how worthless the average first aid kit will be in such circumstances.
From my doctor I also get a prescription for pain meds that do not have a NSAID added.