Good question Clyde.
First thing I can remember writing to completion was a game on the Speccy called the Miner's Strike. It was back in 1985 and I wrote it with my brothers. It was like Space Invaders but with policeman and you fired Miner's at them. It was written in assember and can remember my brother working on the graphics for hours at a time.
The most satisfying is two things.
The first was when I was at school and me and a mate were pissing around and we coded what you call a dot ball. Something like 144 dots spinning around. The teacher was impressed and asked if we could do a head... yeah right! We managed it in the end but looked more like a Mr Potato Head more than anything else.
The other most satisfying, and the one definate memory I have is getting vectors on the Amiga working for the first time. Me and a mate were working for weeks and weeks on getting working! It was something stupid like 4am that it finally worked, a Eureka moment

I jumped around like a maniac and woke the whole house up. We worked on it for weeks and week optimizing it, squeezing every single cycle out of the machine. Eventually we got it faster than TEC/Cryptoburners line source, which legendary as a sort of benchmark for the time.
Approaching my 35th birthday, I find more and more ideas and concepts running around my head. I've literally got a desk full of bits of paper with routines scribbled in pseudo-code and crappy drawings. Never get the time to put them into practice though which is what frustrates me more than anything else.