.
Dijon mustard - Grey Poupon & Maille are in Coto and are not terribly expensive (cheaper than barrio chino). Maille is also in disco/jumbo/vea and is more expensive. No idea about carrefour.s
worcestershire sauce -
I think I've seen Lea & Perrins in all three, coto for sure at the very least.
balsamic vinegar - everywhere. Last year it was even cheaper to buy the real stuff from Modena (http://goo.gl/BFdX1b) than it was to buy the argentine version ( http://goo.gl/mOce2U ). Absurd mismanagement of the economy. Oh well I hit it on a 35% off day and stocked up for a year.
brown sugar - Like others have said, check in the dieteticas, they're all over the city
good chocolate candies - Depends on what you consider good. Disco/jumbo/carrefour have Lindt dark chocolate; I love that stuff, I wish coto would carry it. I also like the little ferro Rocher balls which are available in every supermercado and most kioskos. As for domestic candies, I like the felfort marroc candies and chocolate covered mint patties.
horseradish - There's an Argentine brand which is somewhat ubiquitous.
dill - barrio chino or very high end verdularias. Check out the big mercados like mercado de juramento mercado del progreso etc.
tumeric - Dieteticas. Not all of them but when I wanted some in caballito I only had to aimlessly wander around 5 blocks before finding some
lovage - Good luck! Let me know if you find some.
All of which can certainly be done without but would be nice to find.