I was curious about this - for games that are compilations of titles from other platforms, do you mark the Platform as what you played it on or what the game is for?

I'm asking in a subjective sense since I think an argument could be made either way. This is assuming the port itself isn't transforming the games included, of course.

Example: Would you log completing Mega Man through the Legacy Collection as a [platform you own Legacy Collection on] title or do you log it as an NES title?

I always choose the platform I played it on. In your example, I would choose Xbox One (or whatever). I try to note the compilation in the completion notes, but I don't always remember. Things have gotten very messy with the current generation, especially with Xbox since games have been physically released for both Xbox One and Xbox Series X|S. In those instances, I use Xbox Series X|S, but if I'm playing a game that's not "optimized for Xbox Series X|S" according to the Microsoft store, I log it under Xbox One. The same goes for PlayStation 4 / 5, but Sony seems to separate them a bit more.

Personally, I tend to just mark the platform that gives my completion nice-looking boxart, which tends to be their native platform in the case of stuff like Mega Man. Also, doing so made a cheevo pop, so that's nice. Might change them back now that I think about it, though..

I usually just choose the version of the game I played even if I played it on a newer console, so for example, I played BFBB on the Xbox 360 but since it was the OG Xbox version, I marked it as the OG Xbox. In the case of Mega Man 1, since I beat it on the Xbox One version of the Legacy Collection, I classed it as an Xbox One completion instead of NES.