Bush's unpopularity was going to sink any Republican running in 2008, plus Obama and McCain were both relatively well-liked (Republican hatred of Obama really didn't start until after he became president) so I would say that's the easy answer. 2016 and 2000 were both very close with the popular vote and electoral vote diverging, and the county-by-county maps reinforced the urban vs. rural narrative. 2004 and 2012 were both relatively close reelections of divisive presidents who probably would've won by more in less polarizing times.
2012 was not close by any means. Obama won the national popular vote by 3.9% and won 332 electoral votes. He won every swing state except NC, which he lost by 2%. Aside from Florida, which he won by 0.9%, he won every other swing state by 3%+.
I said relatively, and it was still the closest reelection of an incumbent president in modern times besides Bush in '04. Obama was a lot less popular in 2012 than in 2008 or the early months of 2009 so to me, that leaves 2008 as the definitive answer to this question.
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