Considering Palworld is probably gonna be the best selling game of the year, doesn't have major backing from a huge company like Sony and is only in early access I have to go with it over Helldivers 2. The only thing it has over Palworld is having better legs though its peak is way lower and the lack of a 1.0 launch sales boost could close the legs gap some.

Palworld is pretty damn good game.  Ark mixed with Pokemon.  Not sure what is up with the blind hate by some.  

Palworld by far. Even if it's just because it's a completely new IP, whereas Helldivers 2 is the second game, so at least some people knew about it.

Also, Palworld is a must for the launch of Switch 2.

Sequels to such small games are much less likely to be hugely successful than new IPs (how many sequels, historically, grew as much as HD2?). Still, I'd argue Palworld is the bigger surprise of the two because it was far bigger at launch, and it's a game that I hadn't even heard of before launch.

Both are huge surprises. But i will go with Palword.

Helldivers 2 for me.

I enjoyed the original but was fairly described as typical, while the sequel also looked very similar and generic to many other shooter games out there so why would this have been no different, gameplay and success-wise. But they proved us wrong.

I already knew Palworld would've been a success. It was riding on the coattail of a beloved franchise with its own twisted take. Also the idea of incorporating similar Pokemon designs into their Pal make it more intriguing to many. If it had created monsters completely original then it may not have gathered as much buzz as it did.

Comments that did not age well..

"Has (seemingly) maintained" means I was describing the situation up to that moment in time. It does not imply any prediction about the future. So, the post aged fine. 

"It's easy to see why Palworld would get a lot of attention and then start to fizzle out quickly".

A more recent addition, but Manor Lords has been the surprise for me.
Both Palworld and Helldivers 2 were games that surpassed my expectations, but I expected them to be hits.