Bulettes for some weird reason. I just dig the idea of landsharks, and I'll always work one into the story, usually as an alpha predator of some kind in the area where the PCs are based.
Most recently, I used the bulette riders from Princes of the Apocalypse and had ten mount an attack on my PCs. As soon as I saw that in the book, the encounter popped into my head. It was a cool fight.
Aside from bulettes, because they're more of a random encounter type, I dig mind flayers, also. And have been using gnolls a good deal as well...Yug-Anark, the priestess of Yeenoghu from Gygax's Maure Castle adventure in the 3E days was responsible for the death of a beloved PC, so she's been a major villain ever since. My PCs are working toward tracking her down, so when they're not being ambushed by the Earth Cult, they've been dealing with gnoll warlords.