The 'Figlmüller' is a Viennese institution. For over 100 years, they have offered what is arguably the best Wiener Schnitzel in the world. The portions are humongous, the prices are fair, and the food is to die for. Unlike most other places, they still make their Schnitzel from veal, whereas most other restaurants have switched to pork or turkey.
When you go there for the first time, make sure to order their signature dish - one Wiener Schnitzel with Potato/Corn salad in a pumpkinseed oil dressing ("A Wiener mit an Kartoffel-Vogerl-Salat."). Make sure to bring an empty stomach, and be prepared for some waiting time - the place is almost always overcrowded.