>>35347126He gets fug, but that is all fugs power, Arguably one of the ones without spoonfed Legendaries would be the "Strongest Trainer", Less is more and all that, and even then Sun has Ultra Necrozma to outshine fug.
Nate is definitely up there, but if he qualifies as a "Normalfag" trainer depends on how you see the Musketeers appearing before him, but i always saw him as "Normal" as a contrast to Hilbert/Hildas "Chosen", Runner up in-spite of handicaps.
Sun has both accomplishments(RR and Battle Tree) and POWER LEVEL(Ultra-Necrozma), but got those so conveniently written that makes it more annoying then impressive, but as of now he is probably the strongest.
Until Gen8 because GameFreakl are pissing themselves having the story go "BIGGER SCALE THREATS", which makes each protag look more and more impressive.
Also Calem/Serena is stated to be likely be immortal, so there is also that to consider when talking about "Strongest".