Plot-wise, you should play Fo1 first, that is not a debate.
Then, you can play Fo2, FoT or FoBos in any other. Those three games reference Fo1 events a lot, but don't reference each other at all.
Once you played Fo2, you can play Fo3 or FoNV in any order. They only reference Fo1 & Fo2, but don't reference each other much.
Personnally, i started Fo2 and i deeply regret it. I'll try to spoiler things the least as possible, but there are a lot of mentions of Fo1 main factions and all of those fall flat i you don't know their stuff, as those aren't much present in Fo2.
Also, if you want to really understand some "species", you cannot do it without playing Fo1 as other games don't give them much justice. Sure, it might piss you even more about the way some late games like Fo3 handle those, but at least you would know why.
The more cohesive experience of the bunch is, as mentioned earlier, Fo1-Fo2-FoNV with an honorable mention for FoT. If you are willing to sink time with those three games, i suggest playing those more than once, with different characters and differents choices as you would miss a lot of contents if you only do one playthrough.
For those who played in any order and aren't afraid of spoiler any more, here is an interesting/funny thread :
http://www.nma-fallout.com/showthre...ed-the-Fallout-series-in-the-wrong-order-when
