I'm guessing a lot a people will say power armor, but what's your thought on this?

other than power armor the vault suit/armor and for weapons well I don't know i'll leave that to other members

Clearly the (armored) vault jumpsuit and the hunting rifle. It's in nearly every promotional video since fallout 3 and obsidian continued using the image in NV (see ending slides). Hard to pin anything else as iconic. Besides T51 armor, nothing else makes a recurring appearance and if it does, it usually uses a different model.

T-51.