That title sounded weird at first.
Uhh, I guess Carbink and Plusle are big ones for me. The former is because of my Moon playthrough where I literally only caught a Carbink because I thought it was rare (.... and then the Pokédex explicitly said it wasn’t rare), accidentally giving it a punny nickname and finding that as long as I spammed Sharpen, Harden, Calm Mind and Smack Down/Power Gem/Moonblast/Psychic I’d actually have quite an advantage. The latter is due to a Pokemon X Wonderlocke I played where among Mawile, Garchomp, Gyarados, Mightyena And Rotom, a lone Plusle stood with its Charge and Thunder, and it ended up becoming my secondary lead other than Garchomp. It was also my first death. That was a sad day.