1. Some kind of painkiller. Ibuprofen has good anti-inflammatory qualities that, for me, make it a slight favorite over aspirin or tylenol (acetominophen).

2. A few gauze pads (you can cut them to size), adhesive tape (or, in a pinch, duct tape can be used) and antibacterial ointment (e.g. neosporin).

3. Some way to treat blisters. For me that means a steel needle I can sterilize with a flame to lance them. I often use the adhesive tape (see above) to protect the area. See also: antibacterial ointment (above), in case of infected blisters. Other people like moleskin. Or other products specific to blisters.

4. A small basic first aid booklet, of recent vintage. If it gives much space to making and using tourniquets, it's too old.

5. Any personal medications you take.

It is possible to go far, far beyond these few bare basics I've just mentioned. For example, anti-diarrheal pills are popular, as are anti-histimines such as benedryl. Even antacids. Once you get started, there's really no obvious stopping place apart from personal preference.

Many multi-purpose items can be leveraged in first aid situations. For example, the ubiquitous bandana is extremely useful. Alcohol you brought for your alcohol stove can also become a hand-sterilizer before handling a wound. Duct tape can be helpful if you need a splint stabilized in place.

But by far the best aid in a first aid situation is knowledge. Spend some time visualizing emergencies that might arise and how you might handle them. Take a first aid class. Read a mountaineering medicine book.