I guess Big MT. You've got all those wonderful toys in the Sink and Think Tank. You got plenty of fresh water, and your own interior farm, so you can survive day to day easily. You can teleport to the Mojave and back. You don't need to be lonely, since you've got all your Sink mates to keep you company, but you can also shut them off at any time if you want solitude. There's also a lot of fun stuff to find outside the dome.
There's just one problem...my official Courier never
went to Big MT! He investigated
one mysterious radio signal and got shanghaied into the Sierra Madre heist, he wasn't about to go check any other weird signals anytime soon. The only other DLC place he went to was the Divide, covering the two most hellish places in the game! Even if I can't die, I can't think of a single place in either location where I wouldn't be besieged by the local wildlife at some point in the night. I may be immortal, but I still need to sleep! I guess I'd take the Divide; there's no shortage of working munitions or ammo just in case you finally feel the need to cap yourself.
I guess I could go with my funny character, who
did go to Big MT. It would take a while to adjust to the new female body, but it'd still be a much more pleasant life.
Big Mountain. Only problem is finding where to take a poop.
According to the Think Tank, the blue grass absorbs all nutrients from fecal matter...not that that gives you license to excrete on it.