Depends what's meant by the strongest Ash.
If we're talking about trainer skill then BF and XY Ash are tied for me while Sinnoh Ash comes after them. While I feel like DP Ash triumphs over all the Ash's in terms of strategy, i feel like he relies more on thinking ahead of time compared to the former two who I feel are quicker at coming up with strategies on the fly.
JN Ash and SM Ash come after them for me as I feel like they have instances where they show the same competence as the former three, but I feel outside of these instances they're generally decent trainers, but just not on par with the former three. Then in no particular order I have OS plus BW Ash, that said I haven't watched BW since it ended and since I'll be re watching it soon my opinion may change in the future.
If we're talking team wise then JN Ash is on top by default. Next is the AG team since I consider the Battle Frontier to be more challenging than the gym challenge/island trials. I would argue that some of his Pokemon on both his Sinnoh and Kalos teams are probably stronger than some of his BF Pokemon, but I feel that I can't give them the benefit of the doubt. The last two in no particular order are his OS and BW teams. Note that I didn't include the Alolan team as I actually have a hard time ranking them.
If we're talking about the physically strongest Ash then I'm pretty sure AG Ash beats the others to bloody twitching pulps
Edit: I just realized that my smiley face emoji matches my profile pic XD