What game on Switch was the biggest positive surprise for you, and why?

Mario + Rabbids: Kingdom Battle for me by a mile! An XCOM style strategy RPG that had Mario teaming up with Ubisoft's freaking annoying Rabbids? And with GUNS?! Such an out of leftfield idea that had no business being as good as it was. The gameplay was super addicting, characters were charming and funny, music was phenomenal from Grant Kirkhope, and it had some great boss battles along with challenges for those who really wanna see how good they are at these kinds of strategy games. It was fun deciding which Mario character or Rabbid disguised as a Mario character you would choose to take into battle with you, along with choosing what to upgrade in their individual skill trees and weapons. They also added in collectables for those that were looking to go 100%.

The DKC expansion was a lot of fun too! I'm looking forward to the sequel :)

Hades. I didnt expect it to be so good and it is.

Hyrule Warriors Age of Calamity. I honestly expected it to be trash, but it was fun playing with my friends. Couch Co-op games are underrated. Putting up the difficulty made for a decent challenge when it comes to the bosses.

I'm not sure what's really "surprised" me with Switch. I guess the Mario party remake is the only thing that qualifies.

Was nice to see them finally return to the real classic formula. Though Super Mario Party was there best effort in a long time.

Probably Hollow Knight. One of the best games ever crafted, period. Team Cherry should be proud of themselves. I would have never thought a game that costed me 7,49 euros could wipe the floor with 99% of the so called "Triple A masterpieces". What a journey. What a world. What a score. What a delight to play.
It's simply one of those rare games (Undertale being one of the only examples I can think of right now) that has the heart of its creators all over it.

I will always carry Hallownest in my heart.

If we are talking about Nintendo exclusives... Astral Chain also took me by surprise. I thought it would be similar to Bayonetta in terms of combat. Oh boy, was I wrong... But on the good side! Instead of something familiar, I got what it's probably the most original battle system in an action game I've seen in a long time. The amount of crazy stuff you can do with yout Legions is nuts. I felt like I was playing something truly original and unique, a sensation that is quite difficult to get nowadays. Everything is a copy, a variation or the fusion of many alresdy existing things. This... Was actually new to me. I almost forgot how that felt... 

Splatoon 2, didn't play the first as it was online only, I expected to not like the second one as it was focused on online, and I never play online. I was surprised the single player content was great.

There's a lot of great games on the switch but I expected them to be great so no big surprises.

Splatoon 1 wasn't online only, it had a single player campaign and even local multiplayer that could be played totally offline.

Fully agreed @mZuzek. Astral Chain is the best surprise that came from the Switch for me so far. When I first saw it I knew I needed it and would love it and exactly that happened. Astral Chain 2 is the not announced game that I want the most.

Doom 2016. Was NOT expecting that to come to the Switch!