Carry as many tools as you know how (and are willing) to use...
For a couple of years I have carried something from
CruzTools I think I have the Metric EconoKit M2. That and a couple of rags, some duct tape and a little luck/prayer can get you through some pretty major breakdowns. Besides, the quality of tools in the kit is one step above my buddy's son's plastic wrenches and screwdrivers.
What would I carry if I had more room?
1) decent set of metric and standard wrenches
2) couple of different size screwdrivers (damn little carb screws)
3) adjustable wrench (just in case)
4) a small spool of wire
5) a good pocketknife
6) a magnet (I have dropped screws in the grass before)
7) flares
8) flashlight (a LED crank-up one is good, no worries about dead batteries that way)
9) electrical tape
10) a full set of allen wrenches (metric and standard)
That's about all I can come up with right now. Anyone else care to add to the list? I realize on our bikes that standard wrenches and screwdrivers for carb screws are a non-issue but if I see a bike on the side of the road, I don't care who it is or what they ride, I will stop and offer assistance. You meet some really interesting people that way and hey, we can probably all use some good karma every once in a while.