Nintendo deeply understands a catchy tune. Most don't. Which instrument contributes most to this.

I finally picked up a Switch. After looking at the selection of games at gamestop i decided on Hades. After a couple of minutes of listening to the music i decided that this music will never reach the heights of anything ive already got ingrained in my head. Of cousre, i hope i'm wrong. I can't isolate the catchyness to my age at time of introduction to the music. Now i'm wondering and hoping to identify the instrument that contributes most to a song living rent free in my head. Maybe i will seek out games that feature certain instruments for the entire purpose of creating earworms.

I also didn't think the music in Hades was particularly memorable, but Darren Korb (the composer) did amazing work on Bastion and Transistor.

Part of it is music that speaks to you (or the individual), and other part is that Nintendo tracks (for first-party games) tend to not be cluttered, mixed well, and have good understanding of music theory. It really doesn't matter what instrument they choose, though you have to find out which instrument they use speaks to you the most.

To try and answer your question, I think the symphonic keyboard is the main thing used that makes the tunes in Nintendo games, especially Mario games, memorable. For example, the celebrated water stage theme in Mario 64, or the original Super Mario Brothers theme. I think since the composers have been using this method for a long time, they got it down to a science. They also don't compose long tracks, but short repetitive ones that people are quick to remember, which makes them catchy. Most composers did this back in the day of making video games tracks, it's just that Nintendo hasn't stopped doing it. While other developers hire grand orchestras and the like to produce very long and often forgettable tracks as background music.