Hmm, lets see.
Popplio: Good vs Fighting, Fire, Dark, Ground, Rock, and Dragon Trials. Struggles with Grass and Electric trials. Beats 2 members of the Elite Four; weak to none. 7 to 2.
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Rowlet: Good vs Grass, Water, Fighting, Ground, Rock (ehh) and Dragon Trials. Struggles with Fire, Dark and Ghost Trials. Beats 3 members of the E4; weak to 1. 8 to 4.
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Litten: Good vs Grass & Dark trials. Struggles with Water, Ground, Rock & Dragon trials. Mostly neutral to everything else. Beats 1 member of the E4; weak to 2. 3 to 5.
Overall, Popplio ( and its evolutions ) alleviates teambuilding quite a bit more than its fellow starters. Rowlet comes second by a small margin and Litten is at the bottom of the barrel, but nothing a good team composition cannot fix.