Now that there will be no more alpha releases, it's time to declare the best alpha release we had.
The poll mentions some more significant alphas with major changes (but not truly old ones). You're free to vote in the comments for some other version.
I personally love A27 as I've been playing it the most, but will vote A23 as the last "old-school" version of 0 A.D. It has a specific feel that some later alphas lost. Even the artwork is "more serious" to me, personally.
Why I believe A23: Ken Wood stands out as the best release so far:
1) Much leaner on resources and disk size,
2) Artwork that stands out more, has that gritty feel that makes you want to stare at the icons,
3) Fortress produces "unique" champion units for most civs, while others have a special building for them,
4) Much tougher AI, that will end you if you're not careful. This is perhaps a consequence of it being able to produce champions earlier, without having to research a tech,
5) Named in honor of one of the founding members of the game,
6) Unit dialog screen introduced, allowing for more descriptive entities, etc.