reasonably priced ($4), and it stitches the raster charts together for you. Also if you have a bluetooth bridge you can get instrument data on it, overlays GRIBs, etc. It will automatically select and download the free NOAA charts for you. iNavX is a little more fully featured, but costs a lot more I think and doesn't stitch charts. I have both on the ship's Mac as well - I mostly run PolarNavy NS (the desktop version) for raster charts.

On iOS I have:

PolarNavy MX
Navionics
Plan2Nav (Jeppesen/cmap)
Transas iSailor
Garmin Blue Chart

Navionics use to work a lot better before they went all "social media", still its OK. Charts are free once you pay for the app.
Plan2Nav is the only way to get CMAP charts but is otherwise unimpressive. Cheapest way to get the Explorer data for the Bahamas though. Buggy.
Transas iSailor and Garmin both require purchasing anything you want to view, don't seem to offer anything other than their proprietary charts.