There is nothing like having the tools that are adapted to the fastenings on your bike. An example: two weeks ago I was on a control/feeding station for our club hilly ride. We were not officially a repair point but on the offchance I put a multitool and a pump in with the stove and kettle. We had a rider come through with a flash carbon missile with a saddle loose (and no tools, obviously; carbon missiles only use race back-up
). My multitool would tighten up his saddle but the loose seat-pillar required a real (cranked) allen key (or dismount the saddle to get access to tighten the pillar). Fortunately someone else had a little set of allen keys in their car (not bike-related) and equally fortunately the largest one was the 4mm needed for the seat pillar.
What I carry depends on what bike I am riding (loads of spanners for the tandem, including whitworth sizes for seat pillars and clamps and eccentric, multi-size spanner for the wheels and several little ones for the hub brake cables; just a multitool with allen keys and flat and crosshead screwdrivers for the racer). I always carry a tool for the crank bolts (8mm allen key or 15mm tube spanner depending on the bike) and a spoke key (one of two because the spoke nipples vary a bit and my old hexagonal key won't fit all of them; the cheap round one is a bit loose on some nipples as well). On the mono I usually carry a chain tool and a few spare links but not on the geared bikes (although I have come close to needing it on one occasion). I also carry spare spokes!
One of my friends from a neighbouring club always carries a chain tool. He has never needed it for himself but last season he had to use it four times for other riders - the first time in over ten years. We think that modern 10v chains are not as reliable as the older 9v. (I am still on 7-8v so very few problems).
My pump (I only have one) is a telescopic multi-valve job which has a bracket on the racer; on all the other bikes it goes in a bag (so I have to carry a bag, front or back, which will hold the pump). It pumps tyres up pretty much as hard as my (cheap) floor pump.