If you are going to have only one good first aid kit, then it is best, to keep it in your car. The reason you want to have there is because your car is always with you. If you have a leaky faucet you need the proper tools to stop the leak. Same with an injury. If someone is bleeding profusely you need the proper tools to stop the bleeding. Here is a list of the various items you should have in your first aid kit.

Basic Kit:
Sterile Gauze Dressing 4x4 inch individually wrapped, Triangular Bandage for sling, tourniquet or to tie on large bandage, Ace Bandage for sprains, Roll of 1/2 inch wide surgical tape to secure bandages, Q-Tip Swabs, Assorted Bandages incl. butterfly, finger tip, knuckle, etc., Disposable Diapers, use as a large bandage to help control excessive bleeding, Aromatic Spirits of Ammonia used to revive fainting victims, Blunt Tipped Scissors. Tweezers to extract foreign objects from body, Thick Blunt Needle to remove splinters, Tongue Depressor, Eye Pad, Surgical Mask to protect lungs from smoke and poisonous gases, Oral Thermometer, Rubbing Alcohol (isopropyl 70%), Calamine Lotion for itching, Syrup of Ipecac to induce vomiting, Activated Charcoal to suppress vomiting, Jar of petroleum jelly, Aspirin, Tylenol or Motrin, Non-stinging Antiseptic (beta dine, neosporin, etc.), Antiseptic Towelettes, Fels Naptha Soap for poison ivy, oak and sumac, Pepto Bismol for diarrhea, Milk of Magnesia for constipation.
 
Also in your car you should have a Cell Phone, Large Flash Light, Fire Extinguisher, Jumper Cables, Red Cyalume Light Sticks, Chemical Cold Pack, Dramamine for motion sickness, Liquid antihistamine (Benadryl) for anaphylactic shock, Exacto Knife or Single Edge razor blades, Safety pins, Lighter, Snake Bite Kit, Hard Candy (helps reduce dehydration), Non-conductive rope 10 to 20 feet long, White Sheet, Basic tools, Screw Driver, Phillips, Hammer, Pliers, Work Gloves, Extra pair of reading glasses, tennis shoes, change of clothing, CPR mask with latex gloves to protect you from disease and vomit. 
If you would like to get certified in First Aid and CPR or purchase a CPR mask please go to www.cprpros.com