Conventional Multi Tools
Most riders can get by without a conventional multi tool, but they squeeze a lot utility into a small package. Don't underestimate the value of pliers and a knife. Maybe you wiped out and need to bend a disk rotor or chain ring back into place, or reattach a shifter cable with a zip tie and cut the end off. Good multi tool pliers will even be able to remove a splinter. 

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Top Pick: Leatherman Freestyle

Excellent minimalist multi tool. I used one of as my everyday carry for years, until it was stolen... The problem with other small multi tools is they sacrifice usability for quantity. This has a respectable 2.6 inch blade and pliers which are strong and accurate.

Runner Up: Leatherman Skeletool

This is what I use. Same as the Leatherman Freestyle, plus two Philips, two flat heads, and a bottle opener/clip. You can swap out the Philips and flat head bits for hex keys to be more bike-relevant (you'll need Leatherman-specific bits). 

​High End: Leatherman Wave

Top of the line survival/camping tool. Definitely overkill for biking, but if you're looking for a tool for every occasion, the Wave is hard to beat. Plain knife, serrated knife, file, and saw are all on the outside; in other words, you don't need to "unfold" the tool to access the blades. 

The downside is size and weight - roughly 100 grams heavier than a Skeletool.