It depends what I am doing.
Day rides is a multi-tool, tyre levers, patches, pump and a tube.
A tour would be above plus another tube, chain tool, spoke key, chain oil, quick link, spanner with lots of sizes incl 15mm for pedals, scissors, cable ties, tape.
A faster solo ride (600km+ audax or ultra-race) would be day ride plus a couple of things from tour list plus, with current bike, Di2 charger.
I don't carry spare spokes or cables but do a bit of preventative maintenance. I don't carry masses of tools as there is a tendency to pack for what you would have needed on past trips rather than what is probable I'd need on the next one.
Multi-tools not being able to access certain bolts has caught me out before. I remember a Di2 bar end shifter coming loose and, luckily, I had the right size, but there was too much bulk, so needed a bit of bending and forcing. There is an argument for individual keys vs a multi-tool as gives more options (ie 2 ends!).
Ultralight Igor has some good thoughts on tools (vvv towards the bottom of the page):
http://ultralightcycling.blogspot.com/