F1 - meeting the Master, still one of the creepiest villains i've come across in a video game, really disturbing.
F2 - i guess there were quite a few really, but the one that stands out is blowing up the outhouse and seeing the whole town covered in poop.
F3 - exiting the vault for the first time of course, blowing up Megaton, and in a lesser extent finding myself wonderfully lost in the DC ruins, the first time i'd felt like that since Morrowind.
FNV - not much really, mostly a series of disappointments, the final battle was pretty cool though.
F4 - seeing the interiors on the Prydwen and the Institute for the first time respectively, getting lost in the Glowing Sea, and the ending to the USS Constitution quest. i haven't laughed that hard in a game for quite a while. i still haven't finished the main quest so i've got a feeling there's more to come.