For me it's always either
Over or
Phonto. I find Skitch good for arrows and similar notation, but its text features are really very basic and (in my opinion) pretty cheesy looking.
Over had the edge for a while (and one thing it does offer that Phonto doesn't are collections of hand-drawn illustration elements like banners, icons, etc), but Phonto has had some great updates in the last half-year; it's pretty much all I need now. Also, it's incredibly easy to add your own fonts to both those apps, so you're not stuck with whatever crappy fonts other apps might offer.