The best shortcuts are easy to remember, easy to use, and very beneficial. The ratio of effort to benefit approaches zero. The best example? ctrl + c to copy. It's intuitive, one handed, and saves a 2-3 mouse movements and clicks.
The best shortcut you can add on your iPhone is the shortcut @@ for your email. It'll save you over 10 key presses, upwards of 15-20 if you have a long email. If you're like me, you probably type your email pretty frequently - logging into sites, mostly. This shortcut will make this minor annoyance go away, with the same slick typing improvement of auto-complete. Remember - you only have so many keys left in your life.
Just go to Settings.app > General > Keyboard > Shortcuts, and add the shortcut (Pro Tip: add your work/alternate email as a second shortcut with an additional @).
Then, when you have to type your email, just type the shortcut to get your email auto-inserted. This has easily been the best shortcut I've ever added to my iPhone and it really helps put the "smart" back in "smartphone".