It's a brilliant flash of light then comes the shockwave!
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On topic: Fallout 1 is the story I felt most emerged in (my first introduction to the series). None·the·less for me comparingFallout 1&2 is a neck-and-neck race because honestly I can't make a decision, I love them both.
Dark serious story: Fallout 1 = win!
Re playability, goofy stuff & bigger world: Fallout 2 = win!
Fallout 3, err.. it has its moments, there are a few features that are okay. I really liked Tranquility Lane, I thought it was an awesome concept! Loved the cast: Liam Neeson, it doesn't matter he's my all forgiving dad even though I nuked whole towns into oblivion - he's a great actor and to be perfectly honest I also had a weak spot for Moira and her cute little moleratties. However the overall result was all a little childish IMHO. I rather have the monty python style goofiness of FO2 any time.
Fallout NV, I don't know all the aspects because I only finished it once (siding with yesman). But definitely an improvement.. well, not an improvement - just back to basics reputation/karma-wise.
Anyhow, I'm a bit Fallout tired lately but if you ask me which fallout has the most replay value IMO:
1) Fallout 2
2) NV and Fallout 1
3) Tacticts (still in the lead for the second place - lots of tictacs less talk but still enjoyable)
4) FO3 comes last on my list. Finished Vanilla, the DLC's and with most mods but after a while it becomes a bit flat (monotonous no matter what character I create).
I can understand that some like FO3 better & visa versa, it's like reading a book some people only see boring text and others see a whole new world.