In a vacuum, I'd say Exandria, as it mostly* keeps the standard fantasy-kitchen-sink fare of previous default settings, but lacks the outdated concepts or other lore baggage of those settings, by virtue of the recency of its creation: its lore isn't entrenched nearly as deeply as the FR's is, so there's much more room to tweak stuff if necessary. Same concept, newer model, basically.
The issue there is that the Critical Role company might own Exandria as an IP - collaborations are one thing, but I doubt WotC would want its game's central setting to be one that somebody else owns if it has other options at its disposal. Whether that matters to this thread depends on whether it's about what we want out of 6E, or what we might expect. If it's the former, then hypothetical legal quandaries are irrelevant.
(Eberron would also be fine: the only theoretical issue I can see would be that the feel of the world seems a bit further from D&D default: since the other intended changes would topple a few sacred cows already, keeping other stuff mostly the same might make those changes a bit easier to swallow for some.)
*The one major exception being the increasingly-widespread existence of firearms: an edition with Exandria as the default setting would probably be an edition that treats the presence of firearms as the default. Though I suppose the same might be true of Eberron, so maybe it's a wash there...