It's a tough pick between Frieza's 1st form -> 2nd form, or 3rd Form -> 4th Form.
I have a lot of nostalgia for both, but I always remember getting repeats of the episode where the Z fighters first fight Frieza and Vegeta is so cocky. Seeing Frieza in Linda's incarnation act so coy then transform into the hulk was always memorable and a little frightening. Totally unexpected, as well.
The final transformation has the same elements, but I love the calmer voice, the clean body and how utterly Frieza destroyed everyone. It's very lithe and compact but delivers on a flamboyant menace that the other forms couldn't quite carry.