I'd say 1.05b if just playing through the game to get the story. It feels the most balanced and put together, everything works as originally designed and killing diablo actually feels like a huge challenge. WW barb is such a blast to play there.
But for longevity, I find myself generally sticking to 1.11, as it contains the most content but has awful rune drop rates vs 1.13, so has the most replayability and grindiness of all patches. Honorable mention to 1.07 for some of the buggy builds and crafting, but feels like a beta/cheat code.