If we rule out its predecessor to make things more interesting, which past Nintendo system does the Switch 2 feel the most like to you, and can you explain why?

None of them, honestly. It's a handheld. NES-Wii-U weren't. And the Gameboy-DS line were dedicated handhelds with their own games and own UIs etc. So the only thing the S2 can feel like is S1, only with a bigger screen and better specs. Sorry.

Like SNES, in that SNES as successor to NES kind of way, more of the same, but more powerful.

It feels more like the more technically capable Nintendo systems of their day simply because it's able to run "next-gen" third party games, something none of the Game Boy/DS/3DS could do and the Wii-Wii U could not do either.

So basically the comparable you have left are NES, SNES, N64, or GameCube.

Nothing. Switch 1 didn't even feel like a past Nintendo console but an amalgamation of most 90s consoles. Esp with all the shmups and arcade ports. Jury is still out on Switch 2 for me. I own 1 game in 4 months. I see nothing on the horizon either. So if I have to give a comparison. This is like post N64 launch when it was barren for months. Or those long droughts early years for some Wii games not for parties. It's really too early to make any final comparisons.

Mario Kart World, Donkey Kong Bananza, Resident Evil Requiem, Pokopia, Metroid Prime 4 (cross-gen), Yakuza 3 Kiwami, Pokemon Legends Z-A, Hyrule Warriors, Kirby Air Riders is actually not a bad first 9 months on the market in terms of "new releases". Then you have ports of older games like FF7 Remake and Assassin's Creed Shadows and Star Wars Outlaws. The main weird aspect of the system I'd say is Mario Kart and Kirby Air Riders both being cartoony racing games released fairly close together. 

I'd say it's a fair bit better than PS5 or XB Series though through their first 9 months. 

S2 doesn't really feel like anything else other than S1 and even then the are significant differences as every element of Nintendo's prior approaches are present in the platform but the approach utilized around the Switch platforms are unique to them, the situation itself is like SNES following NES in being bigger, better etc... but those are the only similarities as the Switch platforms are more custom to the userbase's playing habit catering to two parallel markets at once out the box.

The software approach and line up is different between S2 and S1 let alone prior platforms as S2 is more handled like a sole primary platform rather than a vehicle for first party titles, in previous platforms the platform was sold to push solely first party titles where as with S2 third party titles are given more of the spotlight to boaster platform value to the point some are even a major release for when they release (RE9 being the main example of this and CB2077 being another) which isn't something you'd see on previous Nintendo platforms which in comparison had schedule's tied solely to the major first party titles. S2's approach is something you'd see more on platforms like PS2 if anything especially as the are on parity releases while S1's approach was more in line with prior Nintendo platforms.

This approach is not by accident it's to cultivate a userbase who going forward know the Switch platform can be a primary or even sole platform for a consumer as if you look at the first 9 months of releases it's overall the best launch year of any platform and it's not even 12 months yet, the year has been better than most platforms best years let alone launch year.

This.  And it is getting really good third party support, which hasn't happened since the SNES.

I voted SNES, but it reminds me both of the SNES and 3DS. However, it's clearly selling well so far, so I have to say more SNES than 3DS.

The Switch was more like both the NES and DS in that it was the beginning of a brand new paradigm. The NES, especially, started out with Nintendo first party games responsible for the success of the whole system (especially if you look at what was happening in Japan from '83 - '85). Eventually Nintendo built up enough sales momentum and positive good will for the NES/Famicom that third party companies hopped aboard, and by the end of it's life the NES had a really great library of both first and third party games. That is very similar to how the Switch went.

Meanwhile the SNES was selling hot right out of the gate, and third party companies were on board right from the get go. The NES had generated so much good will, that everyone (both customers and third party companies) was on board at launch for the SNES. I see this as a similar situation to the Switch and Switch 2. And the SNES still had decent sales during the back half of its life, but it wasn't as nearly as good as the first half. I am still expecting this for the Switch 2, because of game key cards. I'm expecting third party support for the Switch 2 to cool off at some point, since people aren't buying third party, game key card games as much as expected. So, I think Switch 2 hardware will still have some solid sales during it's second half, but not as good as it's first half.

If the question is defined as "Which other Nintendo hardware is most reminiscent of the Sw2 hybrid platform, not including Switch?", then this is the first time I've been in agreement with any stance JackHandy has had.  Trying to say that any other device Nintendo has released is even remotely similar to the switch platform, based on that perpective, is a really hard stretch.

If the OP is asking about how we feel about similar launch sales or how the platforms were game changers for their time period based on what the new hardware brought to the table, or launch window game releases feel similar to another generation, that's a whole different story.

As far as the platform itself, the closest possible stretch is Wii-U and it isn't even in the same ballpark.  You could barely get the tablet to stream a game from the living room to the bathroom, let alone be a portable system that plays console level quality games.  It's not even worth trying to draw a comparison.