What is the best 3rd party port on a Nintendo system.

[Wyrdness]

You mean of all time? Tbh S2 in itself has some of the best ports on any Nintendo with RE9, FFVIIR and Rebirth and Pragmata, various Gamecube version were good as well. RE4 Wii version is one of the best versions. Doom on Switch 1 was an exceptional port also.