Buy
all the stuff packs! You know you got Christmas money you are just going to spend on plum you don't need anyway.
If you really can only buy 1, I suggest you do a forum search for "what stuff pack" and "which stuff pack"; there have been about 5 threads just like this in the past few weeks and they have some very good answers from the community.
P.S. I vote for Laundry Day, but your best bet would be to go to simsvip and look at each of their
stuff pack guides,; look at all of the listed CAS and BB items in each...and pick the one that appeals to YOU. Callout to @simgirl1010 for this advice, because iirc it's her advice from some of the other threads and I think it is the best advice.