There's no perfect first aid kit. Every kit is a compromise between portability (you may need to walk or hike to the victim), completeness (you can't have the whole ER in your car), and simplicity (you need to know how to use the stuff). What you have in your first aid kit depends on (1) what you need to do, (2) where you're going to do it, and (3) what you're trained to do.

Sample Basic First Aid Kit:   placed in a small backpack or one-tray plastic tackle tox
This first aid kit might be good for a car or cabin. Cost for the components will be around $90. Pre-stocked basic kits like this are available from many sources, on the web, by catalog, in outdoor-specialty stores, and at medical supply stores.

Note: where quantities are mentioned (for example "5 knuckle bandages") consider this a "minimum guideline." You may need higher quantities either for convenience, or because you'll be providing first aid for multiple people. It's best to buy in bulk, then resupply your kit from boxes.

Sample Deluxe First Aid Kit:   fits in a large plastic two-tray tackle box.
This is an extensive kit, with supplies for medical emergencies as well as injuries. It requires some advanced knowledge to use all the components (for example, glucose for hypoglycemia, CPR shield for rescue breathing). It can serve as a home first aid kit, auto or boat first aid kit, or group first aid kit (work or camp). Cost for the components will be around $200. Complete kits similar to this one are sold by specialty medical supply stores