Speculation about this has been rampant forever, it seems. There are arguments in favor of several of the "candidates." 483 has a strong historical connection to the beginning of the C&TS. Being a K-36, it shares common parts with the other K-36's. It has "post-federal inspection inception date" (I know that terminology isn't right, but hopefully I'm getting the idea across) boiler. 463 is a very historical engine and, as an aside, has a unique sound and character compared to the 36's and 37's. Having an operable K-37 back on the property would be nice, too.
I will leave the discussion of which is the best mechanical candidate for restoration to others. I have heard that each has its good and bad points (including 492, which hasn't been in service since around 1965).
If I had a personal favorite for a rebuild, it would be 493 that is rotting away in Silverton. I have a soft spot for it because it was the first narrow-gauge engine other than a K-28 that I saw under steam, 40 years ago in 1967. One of my favorite photos on Cumbres is a John West photo from September of that year of 493 leading an eastbound Cumbres turn at Windy Point.
The biggest challenge to rebuild any of them is money. As other threads have discussed, securing appropriations is becoming an ever more challenging issue. My personal opinion is that the C&TS is going have to secure significant non-tax supported funding from private or charitable sources for these purposes. I've spent nearly a decade-and-a-half working in the public sector, and the simple fact is that--over the long term--"discretionary" public funds (like those for the C&TS) are going to continue to dry up. It's just a fact of life, probably for the foreseeable future.