For years I've thought Sanchez was an underrated villain. Although I rank Goldfinger at or near the top, the perception of the Bond villain among the general fans always seems to be Blofeld, who has suffered from Dr. Evil comparisons and uneven portrayals. But he always seems to be the choice.
The Bond-Sanchez relationship is really complex, more so for me the Bond-Trevelyan connection, which a lot of fans seem to hold in high regard, but it just doesn't resonate the way. We're meant to just suddenly accept Bond has a long history with another 00 and all that. It just seems a bit much.
If I can fault anything about it is that Sanchez is too trusting of this stranger. He must not have believed in too much digging, accepting this new guy into the fold, the way Kamal Khan just figures Bond is an adventurer. Did he not mention him to Orlov, who you'd think would know about Bond, although he doesn't recognize him either when confronted on the train. That's one of the more underrated Moore moments, IMO.
Of course, back then they didn't have the resources they do now that could nail somebody like Bond more quickly. Bottom line, the Bond-Sanchez relationship is one of the most interesting in the series.