Mine was 1.2.5. It just seemed/seems a lot more fun to play it than 1.4.7. The terrain generation seems more interesting, and the mountain biomes always had something to enjoy, not just Emeralds. I still go back and play on a 1.2.5 world every once and awhile.
1.7.3 Sure, no water elevators.. had to use a ton of ladders.. Maybe 1.5 would be a better one to choose. Either way, Minecraft really was a different game back then.
I enjoy them all. Although, if i had to pick one i would pick 1.3 (I think that's right) because i loved the temples, and who could forget sandstone stairs =D
1.2.5 for sure. Hated 1.3, caused so much trouble, and it turned MC onto the "adventure" game path. I did really like 1.7.3, thats when MC was in its prime.
I feel like I need to elaborate on why 1.7.3 was so nice, for all you young'n's out there who joined later. First off, Survival. When you were in a cave, and you got hit by a spider, you just lost 1/10th of your will to live. Gone. Ain't coming back unless you've got food.. and you probably only have 4 pork chops max. Now, you just duck in a corner and regenerate after that 18 meter fall. Too much like every FPS ever, in my opinion, and it really makes the world feel a lot more.. less survival. Next, terrain generation. No mountain biomes. You may think "Oh, no mountains? My life is over now." Thing is, every biome could have mountains. You could walk into a desert, and see this utterly massive floating desert mountain that was just one block update away from failing catastrophically. Nobody can resist causing sand to fall. Simplicity. The most frivolous thing you could make was a painting. Everything served a practical use and resources like iron actually required effort to obtain. Also, no dumb debuffs. Hunting pigmen will never be as useful. I'll probably add to this later, when I realize more things that make Beta 1.4+ (Beds aren't useless if you can sleep in them, after all) soo good.