Assuming an EU citizen wanted to grant their spouse EU citizenship as fast as possible, which EU country would be the best to do so? This is assuming the EU citizen will move to that country with their spouse under Article 10, so it excludes routes such as citizenship for investment.
Actually, gaining citizenship (as opposed to simply having the right to reside, work, etc.) might be the one thing where EU law isn't of much help or relevance. I am not aware of a single country with a fast-track to citizenship for EU citizens' spouses, the only way open to that person would typically be the regular naturalisation process.
According to Wikipedia, the most accessible countries would therefore be Belgium, the Czech Republic, Finland, France, Ireland, Malta, the Netherlands, and Sweden, all of which have a five years minimum residence requirement. That, of course, is an absolute legal minimum and does not account for the duration of the procedure nor does it reflect all the other restrictions and requirements (renouncing your previous citizenship, learning the language, holding a job, etc.) that might apply.
Note that in some cases it might be easier and faster to acquire the EU spouse's citizenship, even without residing in the country (that would for example be the case for the spouse of a French citizen). That's not always the case obviously, some countries even have residence requirements before granting citizenship through marriage.