He is by NO means Ash's strongest - I would say he is on the upper end, but still falls short of the likes of Greninja, Charizard, Lucario, etc.

That being said, he is the "baseline" of Ash as a trainer. Ash bases his entire battling style off of Pikachu, which makes Pikachu Ash's ace; Pikachu is indirectly the reason why Ash is bad at raising and using slow Pokemon like Torkoal and Torterra - but excels at using strong and fast Pokemon.

Pikachu being weak is also how Ash began developing his unpredictable battling style - he was loyal to Pikachu from the beginning, and thus had to figure out ways to overcome Pikachu's lack of power and type disadvantages (like against Brock's Onix, and Lt. Surge's Raichu). This ultimately culminates into the Ash and Pikachu we see today, who can go toe to toe against Champion level Pokemon using these same unpredictable strengths that they've developed together throughout their journey.

Pikachu is strong because Ash is good at using him, and Ash is strong because of his strong bond with Pikachu. 10MTB is not the reason Pikachu is good, but is something that makes Pikachu better.