Favorites per series:
Showa Godzilla: Gojira (1954)
Runner Ups: Mothra Vs. Godzilla, Ghidorah the Three Headed Monster, Destroy All Monsters, Godzilla Vs. Hedorah, and Terror of Mechagodzilla
Showa Toho Non-Godzilla: Rodan (1956)
Runner Up: War of the Gargantuas
Heisei Godzilla: Godzilla Vs. Destroyah (1996)
Runner Ups: Godzilla Vs. Biollante, Godzilla Vs. King Ghidorah, and Godzilla Vs. Mechagodzilla
Heisei Mothra Trilogy: Rebirth of Mothra 3 (199
Millennium: Godzilla, Mothra, and King Ghidorah: Giant Monsters All Out Attack
Runner Up: Godzilla Tokyo SOS
Reiwa Era (Shin & Anime Trilogy): Shin Godzilla (2016)
Runner Up: None...the Anime Trilogy wasn't very good, though Planet Eater was the best one of the three.
MonsterVerse: Godzilla: King of the Monsters (2019)
Runner Ups: Godzilla, Kong: Skull Island (I've enjoyed them all, what can I say?)
Showa Gamera: Gamera Vs. Guiron (1969)
Runner Ups: Gamera Vs. Gyoas and Gamera Vs. Jiger
Gamera Reboots (Super Monster, The Trilogy, & The Brave): Gamera 3: Awakening of Irys (1999)
Runner Ups: Gamera: Guardian of the Universe and Gamera 2: Advent of Legion
Daimajin Trilogy: Daimajin (1966)
Runner Ups: Return of Damajin and Damajin Strikes Again (I like 'em all!)
King Kong's Entire Film History: King Kong (1933)
Runner Ups: 70s Remake, 00s Remake, and Skull Island