While PC has the better of all three, you will need a better computer (ie: you will in the end spend more then just the game price tag) to get it running at ultra most likely. However the mods are comming to pc first, then xbox one, then maybe to PS4. Also, some mods might be blocked from Xbox due to rules and laws.
In the end my sugjetion would be to go for Xbox, then get it for PC at a later date (say a year or so, when you got a up-to-date computer). As for PS4 having better graphics, thats alot of personal preferance. PS4 seems to have a nmore rich but darker color pallete, and xbox seems more washed out, but the difference is so smal, that unless you place two of them right next to eachother, you woudnt notice it.
PS: i am a PS3(4) fan myself.