Mercedes 2025 please.
Here's why. I like RBR, so I want them to dry out massively, become the new Alpine and have Horner leave eventually. Afterwards, they can rebuild and slowly rise to the top again. Verstappen leaving as early as 2025 - to the arch enemy, no less - would be perfect for that scenario to happen. I also like Verstappen, so I want him to have a competitive car to have a shot at the WDC. As of now, I have more faith in Merc than in RBR to achieve that in 2025. Come 2026, Merc will probably come up with a beast of an engine, so Verstappen's chances for future WDCs would be taken care of.
After winning winning and winning and winning with Merc, Verstappen - now the oldest driver in the field - returns to the old nest again for one final season. Post-Horner RBR now have regained some competitiveness, but aren't quite there yet, which is why TV pundit Hamilton argues this will ruin Verstappen's legacy a bit, just like his Ferrari adventure has done to his carreer, respectively. In what arguably will be his best season yet, Verstappen uses all of his old dog's racing tricks (most notably brake testing Antonelli in Saudi Arabia, a nod to 2021) to win WDC no. 10.
You're welcome.
As a sidenote: I hope he won't go to Aston Martin. Such a soulless team.