Which churches should I definitely not miss seeing in Rome?

Yes to San Clemente where you climb down below to see pre Christian era remains plus subsequent ones.
San Luigi dei Francesi had three Carfavaggio paintings as do many other churches in Rome.

Another vote for San Clemente!

If you are in the neighborhood, I like San Pietro in Vincoli. It has Michelangelo's wonderful statue of Moses.

https://en.wikipedia.org/wiki/San_Pietro_in_Vincoli

Plus supposedly the chains that bound Saint Peter.

A few more for ya:
San Giovanni in Laterano is really old and important, and the Pope's church as Bishop of Rome (he wears multiple hats). It's also one of the 4 major basilica churches.

Santa Maria in Trastevere: another very old lady, and has impressive mosaics

Santa Maria Maggiore: another of the 4 major basilicas in Rome; more impressive mosaics; Gian Lorenzo Bernini and Pauline Bonaparte Borghese (Napoleon's sister) are buried in this church; glorious Borghese Chapel.

Santa Sabina: yet another very, very old lady. Up on the Aventine, she's a personal favorite: read up on this one before you go.

Santa Maria Sopra Minerva: built atop of 3 Roman temples, one of them dedicated to the goddess Minerva, thus the name. The little elephant with the obelisk on its back in the piazza has an interesting story.
https://www.througheternity.com/en/blog/art/bernini-elephant-piazza-della-minerva-rome.html

Santo Stefano Rotondo: an old church (Lordy, they're ALL old!) built over a 2nd century Mithraeum (closed to visitors). Due to it's circular shape, it likely was originally meant to be a shrine, and the surrounding walls are decorated with 34 16th-century scenes of martyrdom in all of its ghastly forms. It's not far from San Clemente and San Giovanni in Laterano so it's easy to combine visits to all 3 on the same day.

Read up on these three:
San Pietro in Vincoli. (Oops, I see Pam already mentioned this one)
Santa Pudenziana
Santa Prassede

If you are in the neighborhood, I like San Pietro in Vincoli. It has Michelangelo's wonderful statue of Moses.

I agree, this was worth the steep walk. We arrived at a perfect time as professional photographers had set up their perfect lighting. Our photos were fantastic! Otherwise there are times when the natural light will work nicely.

Santa Sophia Minerva and San Clemente

Sant’Ignazio has incredible frescoes.

Thank you, all! I think one full day of church hopping is in order.

Sant’ Andrea dellla Valle.
It’s lovely inside, all blue and gold, and at the front is an image of St Andrew who was crucified upside down, hence the Scottish flag: the St. Andrew’s Cross. 🏴󠁧󠁢󠁳󠁣󠁴󠁿
I like the little church to the left and slightly behind the Trevi fountain, if only to get away from the crowds there for a bit of peace.

Santa Prassede for the ancient mosaics.

We tend to go into nearly every church we see, as you just never know what surprises await.
Music, relics, paintings, floors, sculpture, etc.
Some are a total revelation!

if you like caravaggio:

Basilica di S. Agostino
Basilica di S. Maria del Popolo
San Luigi dei Francesi

But, be careful that you don't get churched out. We spent a day in Rome seeing churches, and by the end of that day, we couldn't stomach the thought of another church. That was in 2005, and we still limit our church visits to the absolute must-sees, and not all of them.

But, be careful that you don't get churched out. We spent a day in Rome seeing churches, and by the end of that day, we couldn't stomach the thought of another church. That was in 2005, and we still limit our church visits to the absolute must-sees, and not all of them.

Good advice! Many art pieces that I pursue are of course in religious settings. I have to make sure that the most desirable are visited before my partner exclaims "no more churches!".

Thanks, alomaker. My husband loves Caravaggio, so I think we'll prioritize those.

There are a bunch of churches that are on or right by RS “Heart of Rome” walk, and I think they’re all worth checking out. I found them all unique and interesting, whether or not they held a Caravaggio (of which there are several, and some of his most famous/best works.)

I didn’t see Santa Maria della Vittoria mentioned. It’s quite beautiful, opulent Baroque style and is home to Bernini’s “Ecstasy of St. Teresa.”

Bookmarking this post- thanks everyone for the suggestions!

Thank you everyone. My list has actually grown instead of being pared down lol Oh well, it's Rome.

@roubrat. Agree! 😊

The Church of St. Lois of the French near the Pantheon has some amazing Caravaggios.

Church-hopping wouldn't be my choice. They often have odd hours, closing for long lunch breaks and/or masses and, as happened to me in April, a funeral. Instead, note which churches are near other sights you want to see and pop in when possible.

Haven't been yet, but Santa Maria in Cosmedin has the "Mouth of Truth". Planning to stop in October and put my hand in the mouth to see if it gets bitten off for being untruthful.

Considering that you may have The Borghese Gallery on your list , Santa Maria Della Vittoria , would be worthwhile for Bernini's " Ecstasy of St. Theresa " This video is a must before your visit . The Rome you will visit is deeply influenced by the work of Bernini - https://youtu.be/Wet-VF2bDCA ( in four parts , 15 minutes each )

These 6 masterpieces by Caravaggio must not be missed (at Santa Maria del Popolo, Sant'Agostino and St. Louis of the French) https://civitavecchia.portmobility.it/en/itinerari/half-day/how-see-caravaggio-rome-free

Here’s a definitive Bernini guide to Rome https://www.wantedinrome.com/news/where-to-see-bernini-masterpieces-in-rome.html many best of which are his fountains in the Piazzas..

Watch these Caravaggio and Bernini Power of Art episodes outstandingly produced and narrated by Simon Shama before you go..
https://www.youtube.com/watch?v=CX_KWIvIVM8 and https://www.youtube.com/watch?v=dJsD8mmWjM8

In exactly one month my treasure hunt begins…buona caccia!

Not meaning to be a curmudgeon , but the link to the Power of Art episode on Bernini that Ansi12 posted above is chopped short by several minutes . It misses Schama's climactic remarks , so use the link I provided above . Ansi12 , no offense , I'm sure you will understand .

None taken @steven. Next time I’ll quality control which of the dozens of google search results to use lol…

Thank you for the videos! I will definitely bookmark these to watch.

ansi12, please report back on your trip. I have >3 months until I go.