Most Conservative TPG For Classic Gold?

Based on my experience I'd say PCGS. They refused to cross three of my gold coins that I submitted in ICG, PCI, and ANACS holders (1893-CC $5 Liberty graded MS-61, 1853 $1 gold graded MS-64, and 1913-S $20 St. Gaudens graded MS-61, respectively). They did cross my ICG 1838 $5 coin in AU-55. I have an 1852-C $5 in NGC plastic, graded AU-58 which I think got an optimistic grade; it looks more like an AU-53 or maybe AU-55 at best. I also cracked out a 1914 $10 coin from a PCGS MS-61 holder and submitted it to ANACS, where they called it MS-62.

Another reason I prefer PCGS is that I think they are the most likely to reject coins with defects like cleaning, damage, and altered surfaces. If a coin has made it into PCGS graded plastic it probably would not be called a problem coin by any of the other services.