Skeletons.
Being caught in crossfire from multiple skeletons.
Being sniped at by skeletons I can't even see (seems to happen a lot when I got outside in bad weather)
Being knocked back by skeletons as I try to get into melee range if they catch me with no bow.
Being pinned down then killed by a skeleton up above me... somewhere... in the dark.
Yeah, recently skeletons have been scoring quite a few kills on me. It used to be spiders, but I've got better at handling them recently, and my current architectural mood has lots of narrow gaps that they can't get through. I've yet to encounter a skeleton riding a spider - though I did encounter my first ever Slimes a couple of days ago, which was cool, if momentarily nerve wracking when I heard an unfamiliar noise and had no idea what was making it.