How hard is it to get into New Vegas?
If you're going to play just one Fallout game, make it this one. It's easy to get into and you can make literally any build you want, they are all viable(can't be said for the other games in the series). If you like the game, I suggest playing the oldschool ones(1 and 2), altho they are much harder and unforgiving. You will hear about the events, people, factions and lore from the first two aplenty in New Vegas, so they are more for flavor than anything. If you choose not to play these games, Many a True Nerd did a good playthru of both of them, he is also more of a F3/NV fan than an oldschool one so the playthrus are also from a similar point of view as what you would get.
If you liked the gameplay of NV, F3 offers the same thing, in a much more rudimentary form(almost like a tech test of what would become NV). The only thing it has over NV is the exploration, you genuinely want to explore the world and look into every nook and cranny, where as NV has a very boring world that doesn't have much in the way of random encounters(mostly static ones), where as you never know what will happen in The Capital Wasteland. Best reason to own F3 and all it's DLC is to play The Tale of Two Wastelands, which is both games in one(Fallout 3 with gameplay and items of New Vegas, and ability to bring all the gear from F3 into NV and vice versa).
I would not recommend Fallout 4 or 76. They are Bethesda slop that resemble Skyrim more than a proper Fallout game. Fallout 4 can be fun at times, don't get me wrong, but it's simply not Fallout: It's Skyrim with radiation and guns(or a slightly less shitty version of Starfield, you decide)
For an oddball choice, there is Fallout Tactics, a squad based strategy game with elements of RPG that plays similarly to oldschool Fallout. Strictly for hardcore veterans that played both OG games, as the combat builds up from the foundations those two have build and the game requires you to understand the RPG mechanics behind them, as you will not only control one character, but 5 others as well, in real time combat as opposed to turn based if you choose.
Like I said, NV is clearly the best one, and short of playing the original two titles, the one you should play first. Recently, the game got a lot of flak from Bethesda fanboys, jealous of it's success and legacy, but don't let that fool you, it really is one of the best western RPGs ever made. I suggest playing vanilla with all DLC for the first playthru, then going to town with mods for a second playthru, and once you play Fallout 3 and modded New Vegas at least once, move on to Tale of Two Wastelands(which not only accepts all NV mods, it also has quite a few TTW unique mods as well. Theoretically, any and all F3 ones could work as well, but since they were made with an older version of the engine, they require quite a bit of fiddling around with the GECK to make work, strictly for Bethesda modding veterans). Oh, and OG Fallout 1 and 2 both have their own overhaul mods that restore cut content and fix the many bugs, No Mutants Allowed forum is your friend there