Pete Alonso, Mets (nine home runs)

It was known Alonso had some pop as he hit 21 home runs in 67 games at Triple-A in 2018. He was far from the most-hyped rookie entering 2018 though (Vladimir Guerrero Jr. and Fernando Tatis Jr. were talked about much more), but Alonso had a nine-game stretch earlier in the month in which he hit six out of the park. The 24-year-old never appeared in the majors prior to this season.