Rüdesheim, St. Goar or Bacharach

Of the three I prefer St. Goar - central to Rhine destinations both north and south, more scenically located than the others, convenient to Rheinfels Castle ( a short uphill walk) and also to the east bank town of Rüdesheim (from St. Goar, cross Rhine by ferry to St. Goarshausen, catch direct train south to Rüdesheim) and to Marksburg Castle as well (direct train north from St. G'hausen.) There are several hotels with fine views of the river scenery (not so in Bacharach, where the town sits well back from the river.) It's also a good place to end a Rhine cruise.

Marksburg Castle, a river cruise, and perhaps the chairlift ride in Boppard (just north of St. Goar) are among the top things to do.

Can't answer for the other two but we enjoyed our day in Rudesheim. Siegfried's Mechanical Music Cabinet museum was unique and the gondola ride up to Neiderwald Monument over vineyards was so cool. We happened to be there on a beautiful weather day and when their annual celebration was going on complete with costumes and exchange of canon fire across the Rhine. Excellent lunch at a biergarten complete with Rudesheim Kaffee.

Having stayed in Bacharach and visited Rudesheim and St. Goar, I would recommend Rudesheim.

The highlight of that part of our stay was biking along the Rhine from Bacharach to Rudesheim. I'm sure it would be just as much fun the other way around.

I would pick Eltville. Only a few minutes by train from Rüdesheim, it has tons of half-timbered buildings, a lovely river promenade with no train running by, lots of wineries and great restaurants, castle towers, ancient church, and the near-by Eberbach Monastery is a must in my opinion for anyone visiting the Rhein. Take the once an hour bus from the train station to visit Eberbach.