<div class="bbWrapper">Bijuu/susano<br />
Wood<br />
Sand<br />
<br />
I assume we are just talking defencive seen as the OP mentioned "the strongest of the 4"</div>
well featwise, shinsuusenjus body is more durable than PS and kurama even when enhanced by six paths senjutsu.
narutos asura avatar fuses three kurama avatars, so it could potentially be the most durable, but it was destroyed in the only instance where its durability was tested so any claims on its durability are mere speculation. the only thing that can be concretely validated is that it is certainly more robust than a single kurama avater.
gaaras sand doesnt compare to kuramas avatar, PS or mokuton.
Confirmed in Boruto movie along with Sasuke also being faster and superior.
Naruto covered his chakra around people and the enemy kept sending blows towards it and then it was about to burst then sasuke came and put his susanoo head around naruto's kurama chakra head and it held up.
Bijuu PS and Indra's Arrow were both destroyed by the same attack and Sasuke and Naruto received an equal amount of damage, thus their avatars soaked up an equal amount of damage, thus their Avatars are equal in durability with the boosts.