I honestly don't think there's a wrong choice among those three. The UHX version of the 121 I don't think I've played yet, but the 121R has been one of my favorite new uprights for that price point (which was a lower one than the others mentioned here). The list prices on the Walter have increased a lot in the last decade - are the offered prices similar among these?
I like the Walter because it outperforms its size and it's a small, American, family-owned outfit. The looks can be a little polarizing, as the exterior design can look like a 1970s console in a way that doesn't appeal to me for some models. Warranty is transferrable. You have the choice of two different action suppliers.
I like the Kawai as being a well-balanced instrument at typically an attractive selling price/value. Other pluses include the composite action (so long as you like its performance), the transferrable warranty, and it's a well-known brand on the resale market.
I like the Ritmuller because the company (who produce a ton of pianos and have an enormous factory) seems to be trying really hard to break into higher-end production with certain product offerings, and are definitely among the more successful attempts coming from a Chinese factory. I might expect it will have a lower selling price than the others you're considering, as the wholesale price is likely lower and the brand is a less established in this market segment.
I have versions of each of these specific pianos listed in the Piano Buyer "Staff Picks" because I was favorably impressed with each.