I had a long ponder and couldn't narrow it to just the one, so here goes, in chronological order, with tales and pics.
Feeler gaugesWeekends as a kid often involved me helping my pop service his beloved sequence of Ford Cortinas. Tappet setting and contact breaker clearances. He let me use these very feelers (at least I think it's them) to check what he had done. This was a step up from simple spanner passing and set me on the road to being a Mech Eng.
Hide and copper malletMy second car (the first didn't last too long) was a wire wheeled AH Sprite with eared spinners. This mallet was obviously a critical part of the tool kit. I'm rather hoping to be using it again before too long. It feels just 'right'.
A set of Elora spannersInherited from my dad. I've handled a lot of spanners in my time and nothing comes close to feeling as well engineered as these are.
A vernier calliperNo story to this one. I can't even recall when I acquired it. No fettler should be without one. Mr Vernier must have been a clever chap.
Centre punchAnother one from my dad. Simple and hugely effective. A joy to use.
Pliers (bog standard)I joined the Maria Asumpta as engineer in '94 and my tool kit had only a crappy set of pliers. When we put into Ramsgate (a town I was very familiar with) I used the ship's trusty bicycle (Mary) to scoot up to the local motor factors and asked that I have a look at his range of pliers. He laid three pairs out in front of me and these were the pick of the bunch, even though they were a couple of pounds cheaper than the most expensive pair. (And when you're on £10/week that £2 is a massive amount.) It turned out he had the prices the wrong way round, and these were the most expensive pair, but hey ho, I've still got them and they're still perfect.
(A brief side note to this tale is that poor old Mary needed re-cabling and there was a bike shop just down the way from the motor factors so I popped in there. It was here that I first encountered STIs. I lusted after a set of those ever after and finally fitted a set to my old Peugeot in about 2005.)
Coloured Allen keysBought relatively recently from the Edinburgh Bike Co-op. Given the number of bikes in our house and how quickly the children are growing this is currently my most used tool by some distance.
Another hide and copper mallet, but a diddy oneVery recent this one. Spotted in an antiques shop in deepest Cornwall a couple of years ago. It was just longing for a home. It fits neatly into my bike toolbox and within a day of getting it back home I was using it when doing some stem changing. A really cute tool.