I'd start with Fallout 4 if you're new to the series, as it's more newb friendly (in a good way). They did away with the antiquated skill system, and the attribute and perk systems are less consequential, so it's more difficult to make regrettable mistakes when starting without any prior knowledge. For example, if you didn't set your starting attributes just right in the previous games, you might find it impossible to choose a lot of the perks you'd want for your build, or you might have to wait a considerable amount of time to equip certain weapons. However, none of this makes the games impossible to complete, so I suppose this wouldn't matter if you don't care about optimizing your character.