When I fly down in a couple of weeks, to stay for 2 months, here are some of the things I am bringing, based on yo-yo-ing for five years or so:
fresh black peppercorns- they theoretically exist in argentina, but are hard to find and stale. I eat a lot of fresh black pepper.
Misc organic spices- things like cumin, tumeric, various peppers- these, again, are cheaper, better, and fresher in the US.
A Global knife- I just like em, use one every day. There are good knives available in BsAs, but they require a special expedition, and are priced at world prices for good knives- certainly not at US internet discount prices.
Saran wrap and ziploc bags. Argentine quality and availability is wanting.
Me, I mostly drink mate, but my wife is bringing a bunch of her favorite teas.
Fancy organic chocolate bars- for treats and gifts, nothing like em in BsAs.
A bottle of decent bourbon- again, I will switch over to quilmes and fernet after its gone, but its nice to have and hoard. ANY imported liquor is fiendishly expensive. Go local, or do without- fifteen dollar mixed drinks get old fast.
We usually bring some smoked salmon, too, doesnt last long, but for our first dinner party, or host gifts, it goes over real big with Portenos.
Other kitchen stuff we have already brought down includes a good 220volt coffee grinder- easily available online in the USA from various export houses in So-cal, nearly impossible to find and expensive in BsAs. Same thing with a manual pepper grinder- the local versions are just no good. Peugot, all the way.
Electronics, obviously- adapter plugs for computers and ipods, laptops, blank CD's, cables- all of these are cheap and easy to find in el norte.
I am bringing a computerized embroidery sewing machine this time, used, shouldnt be a problem at customs, costs $375 on Craigslist, or $2500 USD for the same thing in BsAs, if you can even find the ones you like. In general, many art and craft supplies are not the greatest- I bring brushes, x-acto knives, small measuring tools, hand tools, crochet hooks, scissors, rotary cutters, and things like that whenever I come.
Hard drive with 25,000 or so songs on it, along with backups of my main computer, as a lot of my work involves images and image editing, which is easy to do long distance, IF you have the data. A separate 1 Terrabyte hard drive these days is a hundred bucks, if your laptop dies or gets snatched at a cafe, you still have all your data.
Personally, I generally bring socks and underwear, but I dont find the clothing scene quite as grim as some do here- but I dont wear the cheapest of the cheap in the USA, either. I would much rather have a couple of really good items, instead of a closet full of chinese crap, and I LOVE some of the local BsAs designers, so I find cool clothes here to wear. Being a man, I tend towards stuff like Hermanos Estebecorena, or the sale racks at some of the men's boutiques, or the young kid designers who popup here and there and then disappear- I have some really great clothes I have bought in BsAs, you just gotta look.
Not big on Beauty products, myself, but I am bringing some Grandpa's Pine Tar Shampoo...