So discounting the five(?) speed blue (!) racer I had as a kid (downtube shifter!) - which I'm quite sure I had no appreciation of as no doubt I instead envied friends' Grifters - and then subsequent yellow "mountain" bike which certainly did get many adventures and much use.
Too small for me mountain bike, £80 from a second-hand shop in ~1996, commuted on 2003-2005
I obviously didn't buy it circa 1996 just to use it seven years later. I got it in my first or second year of uni, no doubt as a plan to save bus fare and/or legs in getting to/from Uni. I think it got used a bit in the first couple of years; I remember cycling to and from hockey practice and games on it.
For some reason I didn't use it for commuting for my first couple of jobs out of uni, instead preferring the bus and walking. However, it was brought back to life for my bike+train+bike commute when I moved away from Huddersfield.
Mountain bike from various parts, £FREE from neighbour, commuted on 2005-2006
When that died, I was fortunate that our neighbour was moving and had some bike bits he donated which he didn't want to take with him. The frame was probably quite a good one, although I can't recall the brand, so this was probably the best bike I'd ever owned/cycled on at this point in time.
My Dad's old bike, £FREE from my Dad, commuted on 2006-2009
When the last bike died I couldn't afford to repair/replace bits and it was easier to accept my Dad's donation of his old bike made from a mixture of cast-iron and lead, so it seemed - it was so heavy. I had swapped my bike+train+bike commute for a plain bike commute, but the total distance remained about the same, a paltry four mile round trip.
This bike moved with us to Scotland and, after an initial six month walking commute break when I lived next-door to work, it was used for my slightly longer commute of a seven mile round trip until it died. I tried, and had limited success, eeking out its existence as a single speed...
Carrera Subway Zero, £200 cycle2work, commuted on 2009-present!
...which ultimately led me to get a fixed gear. Single speed / fixed gear is/was perfect for this length of commute. I wasn't greedy on cycle to work and got what I could afford and what I needed.
(Un?)fortunately it also got used when we moved yet further away from work to my current silly commute and remains (mostly) still bloody going to this day. VALUE for money.
GT GTR Series 5 Road Bike, £600 cycle2work, commuted on 2012-2014
As I've said before I didn't want a road bike. I went for the £600 value based on getting a really good fixed gear, but then couldn't and went for this, an entry level road bike with the worst wheels of any bike I've ever owned (which is saying something). In a way, though, I wish I'd never known what it was like to have gears. It was a fast two years while it lasted.