Well the original is pretty far ahead of anything that came after but personally I enjoy;
Mothra Vs Godzilla
Ghidorah the Three Headed Monster
Destroy All Monsters
Godzilla Vs King Ghidorah
Godzilla Vs Mothra
I also like King Kong Escapes and War of the Gargantuas. Also the original Gamera, and the first two movies from the reboot trilogy. A friend to all children. Even those he crushed and burned to death. * cue theme song *