Like a lot of people, I started with 3 back when it first came out. Enjoyed it well enough to give it a second playthrough.
Played through New Vegas and despite the bugginess, I did like it well enough to finish it at least, but I didn't think too much of it afterwards.
Wasn't until about two years later I played New Vegas again. And I fell in love. Something about it just clicked with me that didn't occur the first time around. I ended up playing through the game at least seven times in a row just trying out all the different builds, companions, DLC, endings, ect.
After doing a bit of research and finding that some of the people who made New Vegas were actually people who made the original Fallout games, I decided to check them out as well.
They both took some getting used to, and overall I enjoyed them. But it really wasn't until a few years later that I really began to appreciate and even adore them.
Fallout 1 and New Vegas are neck and neck in being my favorites. I probably have to give the edge to 1 due to it having the tightest overall experience and it having the novelty (for lack of a better term) of being the first. Although New Vegas had great companions and I really enjoyed the story and conflict in that game.