What's your favorite questline to do? Mine is either House or the NCR.

There are many good ones. It may be easier to ask for the worst questline, and even that is hard.

But I have to go with Vault 11. After exploring that place, I think I literally sat there for a few minutes to just let everything sink in.

Legion.
Arizona killer is great fun.

Bleed me Dry (Red Lucy in the Thorn)