I've never played any of the Fallout games… was thinking about picking up FV4 with all the expansions. Are there any must-have mods. For example, is there a UI mod that is just a MUST HAVE as SkyUi was for Skyrim?
In my opinion, Fallout 4 is far and away the best one - but you have to play on Survival mode for it to really feel like Fallout. Also, there are several mods that I consider a must, including all the big Radio ones
Survival can be harsh and brutal - not to mention time-consuming, so I recommend considering some of the Survival mods that make it a little more lenient. For instance, I use the auto-save on transition mod - because I've lost too much time dying without it.
Survival without mods is MUCH harder than the original Fallouts - but it's also a lot more inconvenient in terms of weight and hunger/thirst. It will depend on your personal preferences. I've never particularly enjoyed constantly having to go back and forth - or not picking up good stuff, so I tend to play with reduced weight mods as well.
It's not "realistic" - but it's also not really fun without them.
Yes, you should definitely get DEF_UI as a minimum. I like to customize my UI to 0.6 scale - but that's somewhat complicated.
That said, there's no doubt that the purists will favor Fallout and Fallout 2.
But, really, they're all great games in my opinion. I prefer FO3 modded and FO4 because I particularly enjoy their take on immersion and exploration.
FO and FO2 can get a little bogged down in endless combat and samey buildings. But, to each is own. They do have better writing, though - which isn't too hard as FO3 and FO4 have crappy Bethesda writing for the most part.