It's all subjective depending on the skill of the player and how each character's equipped, but from reading the Dead is Dead threads I wondered what are the places that each player feels are the deadliest to get through?
For me, I've not played anywhere near the whole game, but already there are quite a few:
(1.). Dead Money - in the switching station where you have to run to a terminal to turn the speaker off before your collar goes off, and the terminal has a trip wire that releases a grenade cluster right behind you.
(2.). Dead Money - in the vault where you have to get to a terminal to turn the speakers off before your collar goes off, and the way to get there is by jumping onto a beam then jumping from that onto the walkway. Easy in theory, darned difficult in practice!
(3.). Dead Money - escaping the vault itself
(4.). Comparitively, from the Mojave Outpost to Novac, the Jackal ambush at the Nipton Road Pit Stop, then the Viper ambush just outside Nipton where they're high above you and there are frag mines on the ground. At an early level, with not very strong equipment, can be a real challenge.