For me,
Locke & Key
Hands down the masterpiece. So much better than the TV show.
30 Days of Night
Some of these minis have been pretty good. The first one that the movie is based on; X-Files/30 Days of Night was solid also; few others. I didn't even mind the recent update on the original.
Winterworld
A personal favorite. Really good art and a solid science fiction story.
Ragnarok
Walt Simonson's take on a very different Thor. His art is as good as ever.
Doomsday.1
A good scifi concept mini from John Byrne. I think this was his last book before he kind of retired from drawing.
Star Trek
Many good runs here. The various John Byrne minis earlier on, the Kelvin timeline stuff, even the recent Year Five seems solid. Star Trek/Green Lantern was also a highlight. I've noticed IDW tends to put their better artists on this property.
There might be some others I forget just now. Been meaning to try G.I Joe: A Real American Hero since I seem to recall enjoying what little I read from the original Marvel run a million years ago and this being the continuation, but it's almost 150 issues in so seems like a big chore to start going through all that...