The Good First Aid Kit is the one you have with you. If it is inconvenient to carry, you won't have it with you. Better to have a couple of bandaids, some alcohol swabs, and a little sterile gauze in a zipper bag in your pocket than to have a 30 lb kit at home.
This is a pretty good little kit
, perfectly suitable for carrying in a ruck bag. Any of the items listed might be good to have around.
Here's a more complete kit
, but I don't think I'd want to lug that thing very far.
Here's the list recommended
by the Boy Scouts of America about what to carry as a first aid kit. You'll notice that they recommend you put it together yourself and store it all in a zipper bag.
A first aid kit doesn't need to be large or expensive. You'll probably never need half of the stuff in there anyway.