Drawing App for iPad

Hi everyone,
As above I'm looking for a drawing app for an iPad which works something along the lines of corel draw. I've tried a couple of so called sketch apps most of which I have difficulty just drawing a straight line!
Whilst I have corel on my iMac (through a Fusion sandbox) I also have Canvas Draw, the OSx equivalent but an iMac is not something you can cart around or sketch on the train when travelling to work in the morning. I've tried Morpholio trace but they want $30 a year and I not going there. I've also tried sketchbook but just to do the simplest thing is exhausting just too many steps.

Any suggestions of an app you are actually using would be great.