Shawn Blanc picks Things, my favorite to-do app. Mike Rockwell goes with my now favorite podcast player Pocket Casts.
My obvious choice would be a web browser, but I understand that’s kind of cheating. So my choice would definitely be Drafts, the mighty all-rounder among text editors. I use Drafts to store notes, formulate texts and publish (!) my blog posts, just like this one. My shopping lists are in Drafts as well as transcribed voice notes from on the go. If it were the only app on my device, my iPhone would still be pretty useful.