Ravenna

I went to all of them and couldn't choose a favorite. Except for one (short local bus ride), they're quite close together, so it's not hard to see them all. Ravenna is gorgeous, and the historic area is walkable. I think there's a ticket that covers 5 or 6 of the mosaic sites.

Due to your limited time, I would focus on the ones in town, not the one outside of town (Sant'Apollinare in Classe). Here's what looks like a good self-guided one day tour:
https://delightfullyitaly.com/2013/09/25/ravenna-italy/

I am currently in Venice , having arrived after three days in Ravenna . The mosaics are indeed , all stunning . I would , however , comment on the general impact of them . The obvious star is San Vitale and next door , the mausoleum of Galla Placidia . Not on the combined ticket is the Arian Baptistry , make certain to include it . If you begin at San Vitale , you should be able to fit the listed sites in one day , providing you get an early start , and try to limit your drooling . While the conventional ( Rick Steve's ) wisdom tells you that the site a bus ride ( 5 miles away ) from town can be skipped , I disagree . St Appolinare in Classe was truly magnificent , and you should shoehorn it in , if at all possible . The #4 bus from the train station will take you there in fifteen minutes .

Also , there is one sixth century site in Ravenna that I think you should skip . This is the mausoleum of Theodoric . It has no mosaic art , but does contain a beautiful porphyry sarcophagus , and the building itself is of major structural and architectural merit . Unfortunately , it is a substantial walk from the train station at the opposite end of the city , and the time would be better spent at the basilica in Classe . I did enjoy it though .

Being my usual self, I did the walking tour somewhat backward, starting with the Basilica di Sant’Apollinare Nuovo from 500 AD. I bought my combined ticket there. However you do it, prepare for lots of WOW moments. You will have plenty of time to see them all in a day.

While you're walking around, look for the Piazza John Fitzgerald Kennedy. Ravenna renamed its market square in his honor in April, 1964. Although there are hundreds, if not thousands, of places around the world named in honor of this much-admired US president, I'm always struck by stumbling onto one in Europe.

You can easily see the mosaics on the combo ticket in one day. (I did skip the one at Classe). I would also recommend stopping in the TAMO museum which will give you added info & history on how mosaics were made and let you see how they add the beautiful gold-leaf into the mosaics. I was in Ravenna two days and really enjoyed my time there.

For an excellent gelato break, stop at Papilla, and be sure to try the piadina sandwich that is a staple in Ravenna.