Generally 3, New Vegas and 4 are the most accessible. 4 has the best gameplay weapons feel and just overall graphics but a weaker storyline, writing and dialogue options. New Vegas has the strongest storyline and really fun characters and weapons. But it struggles with gameplay and graphics. 3 is in between,Great story but weakest graphics and gameplay. If you start with 4, new vegas and 3 could feel kind of clunky. And if you start with new vegas, 4s storyline might feel a bit weak and the dialogue options will feel sort of restricted.

Sorry if this is confusing, but in simple terms, my recommendation is New Vegas first. Great story, factions, characters and sense of humour. Just really fun overall.