We've decided to make a quick stop in Ravenna en route to Venezia. (I know, it deserves more than a snapshot viewing, but...)
I've had some difficulty finding websites relating to Ravenna (particularly with detailed maps that include parking areas.)
We would very much appreciate any advice on the top two or three sights not to miss for a two-hour stopover. Thanks!

Try this web site. It's pretty complete. http://www.turismo.ravenna.it/eng/

I did a day trip there last year, and it is a fabulous place (and if you're there for the mosaics, one day is fine). I bought a book at the train station and decided a rough itinerary of which mosaics to see first. When you arrive at the first one, you'll have the option of a pass that will get you in to several of the churches with the best known mosaics, which was a good deal. Most of the major sites are within easy walking distance of one another, and the area is very charming.

I took a quick glance at the wonderful site Tommaso recommended - great resource that can hopefully help you narrow down your top choices (although with a full day you'll hardly need to narrow!).

Thanks for the tips! Will check out the website, Tommaso. Thanks also to Andrea. Still would like more opinions on narrowing my choices for a visit of only a couple of hours. (We're travelling by car from Roma to Venezia, so it's a LONG day.)

Start with the church of San Vitale and the mausoleum of Galla Placidia behind it. You could visit the attached National Museum or work your way across town visiting churches and baptisteries from there, depending on your time and interests.

I assume you will be in a car--stop to see the church of Sant' Apollinare in Classe just south of Ravenna.