It's got to be 2021. This year is already tracking ahead of 2020, which means it is doing better than the Animal Crossing + beginning of COVID lockdown months. With a new hardware model + Pokemon, Switch is definitely going to be higher this year. At the same time, the momentum this year is only slightly higher. Switch's sales growth was really rocketing up in 2019 and especially 2020. It's not doing that this year, so I think this is going to be the peak year.
Next year will be down slightly, but it will still be high (probably 25m+) due to a bunch of solid software releases. That is why anyone saying Switch 2 is coming in 2023 just has no clue. Imagine Nintendo saying this in 2022. "Hey we are having one of the best sales years that any system has ever had. Why don't we kill off this goose that is laying all of these golden eggs...." Nope. That ain't happening. So while I think 2021 will be Switch's peak year, I think every year 2022 and on will only have mild drops in sales giving the Switch a very long life.