None, each have a role in competitive Pokemon.
Stall and more bulky teams heavily rely on status conditions in conjunction with entry hazards to whittle away the opponent. Removing these will completely invalidate a playstyle.
Removing type matchups will remove way too much strategic depth of competitive Pokemon (predictions, team composition and synergy, and general type knowledge) to be worth doing. Doing this will remove any competitive Pokemon scene, both VGC and Smogon.
Removing stat boosting invalidates so many important Pokemon (Volcarona, Dragonite, Gyarados, Salamence, etc.) and, again, dumbs down the game by removing a strategic component.
Removing priority moves will make offensive teams too hard to deal with, as many builds rely on priority to revenge kill certain threats (i.e. Mega Pinsir vs. a boosted Volcarona).
Overall, implementing these ideas will ruin the competitive aspect of the game, and as such I would rather choose to keep each intact.