The answer to these questions is usually "the one you had when you were young and life was easiest". For me that's the SNES; I love everything about that console and even painted my office in its two-tone grey.
Despite that, the Switch is just the best. It's a home console that you can take on the go (or on the couch while someone else has control of the television). You can play with friends on a big screen or take it on the bus. It's had a long lifecycle that has seen some of the very best games ever made across all of Nintendo's franchises, plus tons of third-party and indie games.
The 3DS is in second place, mostly because it's the one portable-only system that Nintendo got right on the first try.