I used to come here all the time when I worked nearby and my boss would buy me meals (Jason, if you're reading this, thank you again for all the awesome meals and great conversation).
This place really puts chain places like Paradise to shame. There is no silver bullet way to make the perfect sandwich, and Dilly's knows this. I love that you can call in your order and have it ready for you when you get there. The patio is also great--I actually enjoy watching the cars and students go by on University around lunchtime.
I recommend the Buena Bella (for my fellow meatatarians and vegetarians alike) but also check out the BBQ Chicken, Southwestern Club, or the Blazing Buffalo (but you'll need to explicitly ask for red onions on that last one). Don't forget to grab a bag of chips and enjoy one of the best sandwiches in Tempe. :)