With my illustration projects I have gone completely digital. Considering that most of what I'm producing is consumed digitally, there are conveniences with using a drawing app (Command + Z amirite), while making it easy for me to keep my work looking consistent within projects - working digitally is the way to go for me. Here is my list of go-to tools for all of my illustration projects:
iPad Pro: I was originally planning on keeping this list of tools specific to applications, but the iPad Pro + Apple Pencil have become so crucial to my projects that it felt wrong leaving them out. From the first time I saw an iPad, I had grand visions of drawing directly on the screen and I tried SO many styli and apps to try to make it work as well as I had hoped. Through all of the experimentation and trials, it just didn't bring the same sense of joy as drawing on paper. The iPad Pro and Apple Pencil came out and changed everything for me. The only thing I miss about drawing on paper, is feeling the texture. Considering all that I gain by switching to digital, it's worth it.
Procreate: Not much to say here. This app just keeps getting better and better and I absolutely love drawing with it. Procreate needs to be on your iPad.
Adobe Illustrator: I'm not afraid to admit it - I LOVE Illustrator's Pen Tool. I've drawn a lot with nothing but mouse and keyboard - happily clicking and dragging my way around the canvas. I also love drawing in vectors too. The ability to scale to any size without degradation has saved me multiple times when preparing files for print. Even though I don't currently have it on my machine, I will always have a soft spot for Illustrator. If I start up my Adobe CC subscription again, Illustrator will be my first download.
Affinity Designer: Designer has been my Illustrator replacement. Though I'm still getting used to the differences (and haven't yet put their Pen Tool through the gauntlet), I am thoroughly impressed with what they've built. It's a great app that does everything that Illustrator does, with no monthly subscription.
Adobe now has an application called Fresco that combines vector and raster drawing functionality. They are also adding iPad apps for Photoshop and Illustrator this year. If you're up for paying the monthly subscription, you've got some great tools at your disposal.