Fallout 3 and Fallout 4 took over the world. The latter in particular, despite being less universally loved than the former, catapulted the post-apocalyptic franchise to an unprecedented level of success. While the series has enjoyed a massive revival since Bethesda took the reigns back in 2008 though, it's actually been the one game they didn't create that has been the best new instalment so far.
Although it was initially written off as a buggy, broken mess (an assessment which was admittedly not that far from the truth), Obsidian Entertainment's New Vegas perfectly blended the astute storytelling of the original isometric releases with the open-world wanderlust and first-person shooting of Bethesda's revival.
They didn't get the credit they deserved, but the spin-off introduced a wealth of the concepts that Bethesda would build upon and receive such acclaim for with their ambitious fourth title.
Consequently, the majority of people still go straight to Fallout 4 when they think about the series, even though New Vegas did pretty much everything better, five years before.