Rhine Valley castle recommendations

Everyone has their favorite for their own personal reasons. Mine is Burg Rheinfels. Have been there a few dozen times and never get tired of it. Walking into the Great Hall always blows me away. I like that you can spend as much time as you like exploring the ramparts, climbing the towers. This place was and is, massive. Book a tour if you like to take you into the tunnels. They are amazing!

I have also been to the Marksburg a few dozen times too and though it is nice, it doesn't excite me. One is stuck on a 45 min. tour and you cannot explore. There are other Staufen castles in Germany that look just like it, same age, etc. and that have also never been destroyed. The Marksburg just happens to sit on the Rhein, so it has great views and the other ones don't.

The church in St. Goar by the station and docks is beautiful too. It got restored a few years ago and is stunning.

Rheinfels is a compelling set of castle RUINS with an interesting museum, 2nd place on my list, if I must make a list.
https://www.youtube.com/watch?v=qxFF80wORNQ

(The intact buildings belong to a snappy hotel-restaurant operation next door, one of the pluses of visiting Rheinfels, I suppose.)

You can wander independently, but NOT through certain parts for which an official tour is required...

"The mine passages and the lower part of the castle are accessible only with a guided tour."
https://www.romantischer-rhein.de/en/a-burg-rheinfels

My #1 is unequivocally Marksburg. NOT that you can compare an intact castle like Marksburg with period furnishings, etc. to a ruined castle. Like pretty much every other such place where sensitive furnishings are on hand (BURG ELTZ, Neuschwanstein, etc.) Marksburg wants a guide to escort you around. No big deal.

Rheinstein (#3) is small, scenically situated, and nice if you're hiking there from Bingen. Not near a rail station though. You can get off some of the cruise boats (on certain tours) right in front of the castle, then walk up.

I'm a fan of Marksburg, too. It has a true medieval feel to it. Rheinfalls is cool in its own way, but requires a lot of imagination. The other two I have not visited.

Rheinfels has a nice guided tour. Stooping through tunnels and "verboten" areas was fun. The guide was very good.
And, you can play and climb around like a kid on your own. I remember REAL kids were given a paper like a scavenger hunt to fill out.

Of the ones you've listed, it's the only one I've been to. Although I've seen other in the Rhein and Mosel area.

Thank you for your insights. Right now I'm leaning towards Rheinfels; while it wasn't my first choice, with it being mostly ruins, maybe there's more too it, like a good museum or something. Plus, being located at St. Goar, it's right at one of the ferry stops, so it would save on travel time. One thing I've learned on my trip so far is not to overestimate your time and abilities.

I suspect that your current plans will provide additional time to visit Rüdesheim, or Bacharach, or Oberwesel on this day trip. These enjoyable and worthwhile towns are simple hop-off visits that can be incorporated easily on a full-day outing.

Marksburg no doubt requires some additional travel, but it is probably doable for most visitors on a day trip from Frankfurt, I think. Braubach is roughly 20 minutes by train from St. Goarshausen, a cruise boat stop right across the Rhine from St. Goar. Morning cruises (1.5 - 2 hours long) drop passengers at St. Goarshausen at 10:45, 11:00, 12:50, and 13:00, which leaves ample time to reach Braubach and visit Marksburg. After Marksburg, trains from Braubach > Frankfurt take 2 hours, but the trip is DIRECT.

For those with limited time, the best way to take in the Rhine is to spend one night there, IMHO, ideally in a town that is well located for their other travel stops (like FRA airport.) You buy yourself a lot of sightseeing time - and avoid a couple hours of travel logistics - when you wake up in Rüdesheim or Bingen, have a leisurely breakfast, and hop on that 9:00/9:15 cruise boat. Visiting Rheinfels AND Marksburg then becomes a viable option.

I think you will find Burg Rheinfels to be much more than a bunch of ruins. It has an amazing Great Hall, towers and walls to climb, plus tunnels (book a tour) and is so massive it kind of blows your mind.