If I had to hit just one for lunch, it'd be Metro Diner.
Satchel's and The Top mentioned are also very good. Crane Ramen and O.A.K (Original American Kitchen) downtown are also really good. Blue Gill near Shand's is another good one.
Dave's New York deli is a ways from campus but great sandwiches. Hogan's is a good local sub/hero joint.
Don't forget the original Gumby's pizza location (my only pepperoni roll fix while living in FL).
And although most Texans wouldn't find it especially noteworthy, La Tienda is probably the best authentic Mexican food I've found east of the Mississippi. Torta Cubano is life.
Also, I'll get crucified for recommending a FL BBQ joint, but I really like 4 Rivers. I like the unique (to Texans) offerings like burnt ends and their original sandwiches, good sides, and it's worth stopping by even if just for dessert at their excellent bakery counter.