I break the essentials down into catagories with an eye on where and when I'm going and that Murphy will be coming with me.

Navigation (IMHO the most important)
1) Map, the CORRECT map, be it a trail map for suburban/semi-rural park hikes or a 1:50,000 topo with magnetic declination drawn in for a wilderness hike. It continues to amaze me that often I'm the only person around with these.
2) Compass + spare (button, zipper pull, etc)
3) Flashlight + spare bulb and batteries.
4) GPS (optional most of time)

Environment
1) Appropriate clothing for where you are going and what you are doing. Don't wear flannel in a heat wave or cotton t-shirts in the rain.
2) Pack extra clothing as required in case of an unplanned overnight or unexpected weather. You don't have to be comfortable, just alert and with all your extremeties usable.
3) Food and water as appropriate. You can always skimp on food, but running short of water is bad.
4) Toilet kit - Trowel, TP, baggie, hand sanitizer.

Emergency - I was once told by a SERE instructor that you cannot have too many knives, too much cord, and too many ways to start a fire.
1) First kit - Tailor as required. I tend to go heavy on moleskin, pepto, and kneebraces.
2) Fire kit - flint lighter + storm matches and your choice of flammable materias. Indulge your inner pyro.
3) Cutlery - Your choice. Just keep in mind that it's has to be able to take some abuse, may have to be used when your cold tired and stressed, and you're carrying it. My standbys are a Buck Crosslock and/or a SAK.
4) Cord - 30 foot/50 lb minimum and weatherproof
5) Space blanket and/or tarp (instant shelter)
6) Iodine tablets.

Rescue
1) Dead mans envelope (who, what, when, where) left in your car for SAR and/or with somebody at home. Not really required for park hikes, mandatory for off trail wilderness hikes.
2) Whistle
3) Mirror
4) Cell phone (optional)

Other:
Analog watch
Duct tape (often wrapped around hiking stick)