There is a sale going on for the next few days where you can pick up either The Caverns of Grimlock or The Abyssmal Furnaces for 33% off. Honestly, I put these two on sale because out of everyone that has purchased the base game, over 75% of those people have also bought The Infernal Dungeons and The Frozen Expanse. But the two on sale have a purchase rate of less than 50%. So if you've been thinking of getting either (or both), now is the time.
I am rather fond of both of them. I mean, I love all my DLC like they were my children. But the Caverns present a unique challenge in managing light, and it has some really cool monsters. Incidentally, many monsters that come with a DLC will show up in other worlds once you have it. So, for example, you'll see MOTHMEN, ATHACHS, and PUKWIDGIES (plus more) in other worlds after you pick up the Caverns.
The Furnaces are cool because there are all these devils and demons, and the fire world presents a challenge for those who lean towards fire attacks. It forces mages to branch out and explore ice and shock. But there are some really cool creatures in there as well. I particularly like SPRIGGANS, because they're small and unassuming. Until they suddenly grow to 3x their size and pack a wallop! But there's also CHAIN DEVILS, BEARDED DEVILS, IMPS, SHADOW DEMONS, plus many more.
An added bonus is that these DLC don't just add worlds and monsters, but add other things like new weapons and armor, plus abilities and skills. The Furnaces adds 30 abilities, like DIVINE PROTECTION, FEINT, TORNADO, DANCING SWORD, GREASE (a fun one), and RAY OF ENFEEBLEMENT. The Caverns adds 16, like BEAR CLAW, SPIRIT STRIKE, RICOCHET, and METEOR STORM. Like monsters, once you add the DLC everything that is added can be used in all worlds.