As the topic title says, do you have a save you play on where you have made a lot of progress over tha past versions of Minecraft? or do you often find yourself starting over regardless of where you last left off??
I find it hard to stick to the world I played the day before so what I've been doing is playing Hardcore in one sitting until I die. If I don't die and stop playing, I end up starting a new world the next time I play because I no longer have any motivation to continue the previous one.
I lost my very first world a very long time go, around beta 1.4.
As for a world I've kept the longest, it was a beta 1.8 creative world that was a custom skylands I made.
I sort of stopped development on that due to boredom around 1.2.5 I think, can't remember exactly.
Some of my friends that recommended me Minecraft still have their Alpha worlds. They keep 2 files, one that keeps updating with Minecraft, and one that sticks on the version the world was created. I think it's Alpha 1.2.4, but I'm not too entirely sure. It's amazing how much has changed when you compare the two worlds, brings a sense of nostalgia.