I already have Dominion, Prosperity, Alchemy, and Guilds. Due to circumstances, I can only buy one expansion among Hinterlands, Cornucopia, or Dark Ages. So what set would you recommend I get next to make for interesting interactions with the current cards I have?
Depends on your preferences:
Hinterlands is an expansion with 26 cards, many of which help you deal with victory cards and other junk. Some of those are sifters, but there's also Trader, which will make you drown in Silvers. It also (and most prominently) introduced the concept of cards that do something when gained, including two that harm your opponents when gained. It is probably the set with the least complex cards. It has an a bit above average amount of victory cards (and treasures). It's a lot more tame than Dark Ages.
Dark Ages has two things going for it: Being a very, very big expansion with 35 kingdom cards and throwing a lot of unwritten rules for Dominion out of the window (well, you can take this as a "con", too). This includes gaining cards from the trash, a pile with not 10 but 20 cards in it, a pile with all different cards, a new kind of junk card (Ruins), cards that can only be gained through other cards, and Shelters, which you may use to replace starting Estates. So Dark Ages is a lot, lot new stuff to learn and not really a "beginner" set. If that's okay for you and you want something "fresh", take this one. One theme of the expansion is the trash, by the way. It has cards that do something when trashed.
Cornucopia is the smallest of the three. With only 13 kingdom cards, you should consider how much content you get for your money. Still, Cornucopia has some very cute cards that are interesting to play. The set's theme is variety, which is why it has cards like Menagerie that reward you for it, or Young Witch, which enforces it (you play with 11 kingdom cards if it's out). Tournament is a card that allows you to gain the 5 unique prize cards that are also part of the set. As most cards in Cornucopia (Hunting Party being a clear counterexample) demand variety, this also means new strategies to go for.
All three are fun. Hinterlands and Cornucopia are easier to grasp, with Cornucopia having a bit more cards that encourage you to play differently than you normally would, but with Hinterlands offering more new cards. Dark Ages is a lot of cards and a lot of rules. It's crazy and changes the game a lot.
Hope this was helpful.
Edit: Didn't resist removing a double negative.
Edit 2: Removed a claim about the quality of the Hinterlands cards.