Much as Dalarast says, it really depends on your personal circumstances. The main thing to decide is what situations you want to prepare for, and then consider what factors might impact that (weather, terrain, etc).
I am a family man, who works 50 miles away from home. My bag is geared towards getting me back there if something goes wrong, or dealing with traffic jams or foul weather that might keep me out for several hours or overnight. I have food and water for 'stuck in traffic' situations (used twice in the last few years, the worst being one where I got home seven hours late), but I also pack for some other scenarios:
Car breaks down in snow. Help unable to reach me due to weather conditions.
Blanket, pee bottle, food and water. Spork. 4 x 4 hour heat pads.
Snow shovel & track mats (throw them under the wheels to help get the car out of snow)
Need to get home, car not an option:
Emergency cash (enough to pay for a taxi, cafe food, etc)
Walking shoes & good socks
Waterproof coat
Wool sweater, hat, gloves and leggings. Kept compressed in a dry bag.
USB power bank (for phone).
Backpack
General gear that I just like to have handy.
Water purification
Fire lighting kit
Sturdy but UK legal folding blade
Light tarp & cord
Map & compass.
First aid kit
Umbrella
Now much of the above may be used for things as mundane as family days out, but gives me options. I keep most of it in the backpack, but the spare clothes, walking shoes and a few other odds and ends just sit along-side that in an old duffle bag.